Abstract Introduction. CLL has a highly heterogeneous clinical course. In fact, while some patients experience a very aggressive disease, others may present with an indolent form, and eventually die of “other” causes. Here we report the findings of a prospective cohort study evaluating the comorbidities and causes of death in newly diagnosed patients with CLL. Methods. Clinical information was collected from 1174 patients with newly diagnosed CLL (within 9 months of diagnosis), who consented to participate in a prospectively observational trial with standardized longitudinal follow-up between 01/2002 and 11/2014. All comorbidities were diagnosed before presentation. Results. Baseline characteristics are shown in the Table. Co-existent health conditions included: cardiac (28% of patients), respiratory (18%), endocrinological (14%), rheumatologic (42%), gastrointestinal (33%), genitourinary (35%), and psychiatric diseases (17%), diabetes (10%), hyperlipidemia (41%), other cancers (20%), stroke (3%), hypertension (40%), venous thromboembolism (3%), substance abuse (5%), sexually-transmitted infections (3%), and obesity (32%). After a median follow up of 4 years, 197 patients died and the cause of death could be accurately determined in 135 patients. Among these, death was due to CLL progression in 69 (51%) patients, infections in absence of CLL progression in 7 (5%), other cancers in 29 (21%), and other comorbid health conditions in 30 (22%). CLL patients (N = 1174)Number (%), median [range]Age (years)63 [23-89]Males791 (67)Females383 (33)Rai stage 0604 (51)I-II512 (44)III-IV54 (4)missing4 (1)B2M < 4 mg/dL717 (61)> 4 mg/dL97 (8)missing360 (31)CD49d negative638 (54)positive277 (24)missing259 (22)CD38 < 30%780 (66)> 30%332 (28)missing62 (6)ZAP70 negative650 (55)positive380 (32)missing135 (13)IGHV mutated506 (43)unmutated394 (34)missing274 (24)FISH del13q395 (34)negative173 (14)+12276 (23)del11q90 (8)del17p50 (4)other2 (1)missing188 (16) Conclusions. Despite the advanced age at CLL diagnosis, the frequent presence of comorbidities, and the indolent nature of the disease, CLL progression is the ultimate cause of death in 51% of newly diagnosed CLL patients, with an additional 26% dying of causes potentially related to CLL, such as infections and second cancers. The influence of comorbidities and other clinical factors at diagnosis (e.g. smoking, Charlson Comorbidity Index) on survival and ultimate cause of death are being abstracted, and will be presented at the meeting. Citation Format: Paolo Strati, Kari Chaffe, Sara Achenbach, Timothy Call, Neil Kay, James Cerhan, Susan Slager, Tait Shanafelt. Comorbidity and cause of death in patients with chronic lymphocytic leukemia (CLL). [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5267. doi:10.1158/1538-7445.AM2015-5267