The objective of this study was to assess the frequency of comorbidities and multimorbidities in rheumatoid arthritis (RA) patients under biologic therapy and their effects on biological disease modifying antirheumatic drugs (DMARDs) choice, timing, and response.Hacettepe University Biologic Registry (HUR-BIO) is single center biological DMARD registry. Cardiovascular, infectious, cancer, and other comorbidities were recorded with face to face interviews. Multimorbidity is defined as >1 comorbidity. Disease duration, initial date of biological DMARDs, initial and overall biological DMARD choice were recorded. Disease activity score-28 (DAS-28) responses were compared to comorbidity presence and multimorbidity.Total of 998 RA patients were enrolled into the study. The mean age was 53.1 (12.5) and mean disease duration (standard deviation [SD]) was 11.7 (7.5) years. At least 1 comorbidity was detected in 689 (69.1%) patients, 375 (37.9%) patients had multimorbidity. Patients had mean 1.361.32 comorbidity. The median durations of first biological DMARDs prescription were 60 (3-552) months after RA diagnosis. For multimorbidity patients, the median first biological prescription duration was longer than the duration for patients without multimorbidity (72 [3-552] vs 60 [3-396] months, P<.001). The physicians prescribe tumor necrosis factor inhibitor (TNFi) biological drugs less frequently than other biological DMARDs in patients with at least 1 comorbidity (66.2% vs 74.5%, P=.007) or multimorbidity (34.6% vs 43.5%, P=.006). Patients with comorbidities and multimorbidity achieved DAS-28 remission less frequently than patients without comorbidity (31.6% vs 42.6%, P=.012 and 27.2% vs 39.7%, P=.001, respectively).In real life, physicians may postpone to prescribe biological DMARDs and less frequently choose TNFi biological drugs in patients with multimorbidity. Furthermore, comorbidity may have a negative effect on the treatment response.