This study examined variations in the diagnosis of depressive disorders and prescription of antidepressant medications in primary care and the contribution of the physicians' practice styles to these variations. The analyses were based on visits to a representative sample of the U.S. office-based primary care physicians from the 1997 and 1998 National Ambulatory Medical Care Survey. In this sample, the propensities to give a diagnosis of depressive disorder and to prescribe antidepressants were operationalized as propensity variables. The association of these variables, obtained from a randomly selected subsample of visits to each physician, with the diagnoses and treatments of the other patients seen by the same physicians was examined. The results revealed considerable variations across practices in the percentages of patients who received diagnoses of depressive disorders (0-25%) and prescriptions for antidepressant medications (0-38%). Furthermore, the physicians' propensities to diagnose depressive disorders or to prescribe antidepressants were significantly associated with the diagnosis and treatment of individual patients. The large variations in diagnosis and treatment of depressive disorders and the significant impact of practice style variables on these variations highlight the need for implementation of uniform practice guidelines for diagnosis and treatment of depressive disorders in primary care settings.