The vast majority of psychologically distressed primary care patients present exclusively somatic concerns at the outsets of their visits. However, it is not known how often such patients subsequently disclose psychosocial problems to their primary care physicians (PCPs) and what variables predict such disclosures. Our objectives were to measure, among psychologically distressed primary care patients, the frequency of disclosure of psychosocial problems (disclosure), the effects of prior psychosocial inquiry (prior inquiry) by PCPs and various patient variables on disclosure, and the effect of disclosure on mental health problem recognition (recognition) by PCPs. The study was based in the practices of 69 community-based PCPs and involved 308 adult patients with 28-item General Health Questionnaire scores of 5 or greater, indicating significant psychological distress. Disclosure occurred during 51% of visits overall and 67% of visits with prior inquiry. The odds of disclosure were increased by prior inquiry (p < 0.001), greater physician-patient familiarity (p < 0.001) and greater severity of patient psychological distress (p < 0.001). Prior inquiry and physician-patient familiarity had a negative interaction (p < 0.05) of smaller size than either variable's main effect, so that their combined effect on disclosure exceeded the effect of either variable alone but was less than multiplicative. The estimated odds ratio for recognition given disclosure was 24.13 (95% confidence interval, 11.28-51.63) after adjustment for the effects of significant covariates. We conclude that if PCPs inquire, most psychologically distressed, somatically presenting patients will disclose psychosocial problems. Inquiry is particularly productive with unfamiliar patients. PCPs can engender a substantial increase in psychosocial disclosure simply by adding one or two questions about mood or interpersonal problems to their clinical interviews.