Abstract Background: Contralateral breast cancer (CBC) is diagnosed in 1-2.6 % of all breast cancer patients directly at diagnosis, after that the risk still remains high (0.7% per year) throughout life of the patient. Breast cancer patients are followed up by palpation and clinical mammography after their diagnosis, partly to be able to diagnose a possible CBC. The aim of this study is to investigate if the diagnostic work-up of patients with breast cancer has improved over the last thirty years and resulted in earlier CBC diagnosis.Materials and Methods: Synchronous contralateral breast cancer is here defined as two cancers occurring maximum 3 months apart, cancers with longer latency time are defined as metachronous. The study population is restricted to patients with maximum three years between the two cancer diagnoses. In this study we used two cohorts; all CBCs in Sweden 1976-2004 (N: 3260), and all CBCs in Stockholm during 1976-2005 (N: 688), the smaller cohort had more detailed information. The study participants were selected from two population-based registers (Sweden Cancer Register and Stockholm Breast Cancer Register). With logistic regression we calculated the odds ratio of being diagnosed synchronously, as compared to metachronously, adjusted for calendar period and age at first diagnosis.Results: The risk of synchronous diagnosis was increased by 27% per each 5-year period, the change was statistically significant. We then investigate if this risk increase by calendar period was due to improved clinical work-up, by including detection mode in the model. We found that the risk estimates for calendar period persisted, thus giving no evidence for improvement of clinical work-up during the study period. This result did not change when stratifying the study population by age at first diagnosis (below/above 55). Next, we instead included adjuvant therapy for the first cancer in the model and the association with calendar period decreased to non-significant levels. We also showed that the proportion of CBCs detected trough clinical work-up has not increased over calendar period (p=0.89), neither when including, nor excluding, screening mammography as part of the work-up. The mean tumour size of the contralateral breast cancer was 16.0 mm and has not decreased over the study period (p:0.62)Discussion: We found an increase in the proportion of synchonous CBCs compared to metachronous over calendar period, this change is not associated with clinical work-up, but with adjuvant therapy. This study gives no indications of any improvement in diagnostic work-up of contralateral breast cancer over the last thirty years. Such an improvement would potentially be very important for survival of the growing population of (first) breast cancer survivors. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 4011.