This article identifies the characteristics of patients and office visits associated with decreased mutual decision-making between physicians and patients. In the baseline cross-sectional survey of the Medical Outcomes Study we measured specific patient characteristics hypothesized to influence participatory decision-making (PDM) styles of physicians. We related these characteristics to the PDM style scores for their physicians. The study was conducted in solo practices, multi-specialty groups, and health maintenance organizations in Boston, Chicago, and Los Angeles. Over a 9-day period in 1986, 8,316 patients were sampled from the practices of 344 participating Medical Outcome Study physicians, representing general internal medicine, family practice, cardiology and endocrinology. Physicians' PDM style was measured using a 3-item scale included on the baseline questionnaire completed by patients after office visits to their Medical Outcome Study physicians. We found that the elderly (age 75 and older) and young adult (younger than age 30) patients, patients with high school education or less, minority patients, and male patients had the least participatory visits with their physicians. We also found that male patients seeing male physicians had the least participatory visits compared with male patients seeing female physicians, and compared with female patients seeing physicians of either gender. Our data indicated that PDM style increased as duration or tenure of the physician-patient relationship increased. Participatory decision-making style also increased with increasing length of office visits. The role of effective interpersonal care in optimizing patients' health outcomes may be underappreciated. We have identified seven patient and visit characteristics that maximize or compromise the effectiveness of interpersonal care. Recognizing those at risk for suboptimal interpersonal care may be a first step in improving the management of chronic disease. Key words: participatory decision-making style; interpersonal care; doctor-patient communication.