A prevalence study carried out in Berlin aimed at determining the incidence and distribution of psychiatric diseases among the patients treated by a general practitioner. A questionnaire had to be completed by the patient and by the physician at every consultation. This form comprised not only questions regarding the patient's main concern and self-assessment in respect of physical and mental complaints, but also a list of actual signs and symptoms of physical and mental disorders as well as questions on demographic and social variables. The physician was asked to state details on findings, principal and secondary diagnoses, kinds of therapy and also the time the consultation lasted. The results show clearly that psychiatric diseases occur in the general practitioner's consultation-room and in his practice in a proportion that is large indeed; and that the patients' self-assessment regarding the predominantly" or exclusively" mentally conditioned nature of their complaints and the formulation of their main mental concern of frequent affirmative answers in the list of mental complaints largely correlated with the statements regarding mental or psychiatric findings or principal diagnosis made by the physician. There were also clear interrelations with demographic and social variables, particularly in connection with the kinds of therapy that had been selected. On the whole, this study showed once again clearly that mentally diseased patients do not receive all the satisfactory and appropriate treatment of which they stand in need.