Abstract Background Rarely, Legionnaires’ disease (LD) can progress into a slowly or nonresolving form. Methods A nationwide retrospective study was conducted by the French National Reference Center for Legionella (2013–2017) including cases of slowly or nonresolving LD defined as persistent clinical symptoms, computed tomography (CT) scan abnormalities, and Legionella detection in lower respiratory tract specimens by culture and/or real-time (RT) polymerase chain reaction (PCR) >30 days after symptom onset. Results Twelve cases of community-acquired slowly or nonresolving LD were identified among 1686 cases of culture-positive LD. Median (interquartile range [IQR]) age was 63 (29–82) years. Ten (83.3%) patients had ≥1 immunosuppressive factor. Clinically, 9 patients transiently recovered before further deterioration (median [IQR] symptom-free interval, 30 [18–55] days), 3 patients had uniformly persistent symptoms (median [IQR] time, 48 [41.5–54] days). Two patients had >2 recurrences. CT scan imagery found lung abscess in 5 (41.6%) cases. Slowly or nonresolving LD was diagnosed on positive Legionella cultures (n = 10, 83.3%) at 49.5 (IQR, 33.7–79) days. Two cases were documented through positive Legionella RT PCR at 52 and 53 days (cycle threshold detection of 21.5 and 33.7, respectively). No genomic microevolution and no Legionella resistance to antibiotics were detected. The median (IQR) duration of treatment was 46.5 (21–92.5) days. Two empyema cases required thoracic surgery. At a median (IQR) follow-up of 26 (14–41.5) months, LD-attributable mortality was 16.6% (n = 2). Conclusions Slowly or nonresolving LD may occur in immunocompromised patients, possibly leading to lung abscess and empyema.