Understanding the role primary care physicians play in the recognition and management of mental disorder among their patients is a research topic that has assumed considerable importance because of the high prevalence combined with low recognition of mental morbidity in primary care settings. Patient characteristics that influence diagnostic patterns have been one important focus of this research. This paper presents data from a study in two primary care departments in a large comprehensive health care clinic in central Wisconsin. During a 3-month period, 1,452 attenders at these clinics were assessed using a brief psychiatric screening scale, and special study forms were completed by the primary care clinicians. Consistent with previous research, reason for visit, psychiatric symptoms, and prior knowledge of the patient are significant predictors of physician diagnosis of mental disorder. Additionally, pattern of physician practice, as represented by differences between the two types of clinics, was a strong predictor of both diagnosis and treatment. The lack of significant effects of sex and previous medical utilization is contrary to previous research. Key words: mental disorders; diagnosis; primary care.