Abstract Introduction: Multiple myeloma (MM) is malignancy of plasma cells, which secrete monoclonal antibodies that are detectable in the patient's (pt) serum and/or urine. Infrequently, MM may present as an oligosecretory disease, where monoclonal protein (M-protein) and involved free light chain (iFLC) are either not detected (=non-secretory) or are both below the threshold for measurable disease (=oligosecretory) as defined by International Myeloma Working Group (IMWG). The incidence of non-secretory MM at presentation has been estimated at 1-2% [Chawla, Eur J Haematol 2015], yet data regarding the frequency and clinical phenotype of oligosecretory relapse is lacking. These pts are typically excluded from most clinical trials. Methods: Pt's MM was classified as oligo-secretory, in the absence of measurable disease according to the IMWG criteria (M-protein≥1 gr/dL, or U-PEP > 200 mg/24 hrs or involved free light chain≥ 100 mg/L). Relapse was defined according to IMWG criteria, based on changes in monoclonal protein in the serum or urine, bone or extramedullary lesions on imaging, bone-marrow plasmacytosis, serum hemoglobin, creatinine and calcium levels. Pts treated at our center for MM, who had secretory (i.e., measurable disease) MM at diagnosis, and relapsed (secretory or non-secretory relapse) between January 2016 to July 2020, were included. MM baseline pt and disease characteristics, disease characteristics at relapse, treatment regimens and outcomes were documented. The first oligosecretory relapse (OSR) that any given pt experienced was defined as the index OSR for that pt. For each pt with an OSR, we identified the first 4 pts with a secretory relapse (SR) in the dataset, who matched the pt by the relapse index number and calendar year of relapse, to form a SR comparator group. We compared pt and disease characteristics, therapy patterns and outcomes between the OSR and SR groups. Results: One hundred and seventy-seven pts with relapse were identified; 8 (4.5%) had oligo-secretory disease at MM diagnosis and were excluded; 152 of the 169 pts who were secretory at presentation (89.9%) had secretory MM at all relapses; 17 (10.0%) had an OSR (4 non-secretory and 13 oligosecretory), the SR comparator group included 67 pts. Pts with OSR had similar characteristics compared to SR pts at MM presentation, in terms of demographics, FISH cytogenetics, ISS, levels of M-protein and involved FLC, frequency of extramedullary disease, target organ involvement; Treatment pattern and response to upfront therapy were comparable (Fig1 A). Oligosecretory disease was more frequent at relapse compared to newly diagnosed MM (10% vs 4.5%), proportion of OSR among pts with previously secretory MM increased in later relapses. The proportion of OSR from total 3 rd or 4 th relapses, was high as 20% and 17.6%, respectively (Fig 1B). OSR pts had a higher rate of new plasmacytoma (53% vs 9%, p<0.001) as the criteria for relapse, and a trended towards increase in LDH, and higher rate of extramedullary disease (17% vs 4.4%, p=0.09) whereas increase in monoclonal protein was more frequent in the SR group as a criterion for relapse. Overall response rate to therapy of the index relapse was similar between groups among evaluable pts (58% vs 64%), however, in 5/17 (29%) of the OSR, response was non evaluable from available documentation. Median follow up was 10.2 months [Q1 4.1- Q3 16.7]. Twelve-months progression free survival was 82.4% vs 73.8% (p=0.76), and 12-months overall survival was 60.2% vs 64.75% (p=0.60) in RS and OSR, respectively. Conclusions: Oligosecretory disease was more frequent in relapsed MM, compared to its rate at MM presentation, reaching 10% of the pts with relapsed MM and increasing in more advanced relapses. Pts with OSR and those with SR had similar clinical characteristics of their MM at presentation as well as comparable outcomes, but pts with OSR had higher rates of new skeletal and extramedullary lesions. As identification of the OSR may be challenging in the absence of serum and urine biomarkers, awareness and clinical alertness are warranted to avoid end organ damage. We suggest inclusion of OSR pts in clinical trials should be considered, despite some challenges in following their therapy response, as they comprise a non-negligible proportion of pts, in particular in the advanced relapse setting. Figure 1 Figure 1. Disclosures Avivi: Kite, a Gilead Company: Speakers Bureau; Novartis: Speakers Bureau. Cohen: Neopharm / promedico: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Karophram: Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; GSK: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees.