Primary herpetic gingivo-stomatitis, a well -defined clinical entity, appears to present a peculiarly difficult diagnostic challenge to the family doctor. Possible reasons for this have been examined in a series of studies over a period of three years by one general practitioner in a large group practice. Analysis of the early clinical manifestations of the disease, studied in a prospective survey, showed upper respiratory catarrh to be a prominent feature. The difficulty of spotting this and other early features of the disease against the background of similar presentations of different - and more common - illnesses was demonstrated in a one -year prospective survey of all febrile children seen by this family doctor. Laboratory support for the main clinical survey led to antibody studies of affected children, of a control group, and of mothers of both groups. The findings supported the clinical diagnosis, confirmed the high prevalence of infection in the mothers, and provided information on some changes in antibody response in relation to the length of the follow-up period. Recurrent clinical infections and possible sequelae were already evident, even within the relatively short duration of the survey. These phenomena, examined against the control group employed in the antibody studies, included herpes facialis, various infections, and recurrent mouth ulceration. The findings were compared with results of a retrospective study of cases of probable herpetic stomatitis and of control children, selected from the group practice records. Similar results were obtained in this survey. Comparison of households containing affected children against others from the practice showed the disease occurred more frequently in a setting of poor hygiene and larger sized families, factors which also hinder the family doctor in diagnosis. From the pattern of admission diagnoses of cases treated in the Edinburgh City Hospital during the three years of the studies, it was clear that other general practitioners in the area shared difficulties in diagnosis. The results of all these surveys have been examined in the light of published reports, with particular reference to diagnosis in general practice. The differential diagnosis has been discussed and illustrated by examples drawn from the day-to-day work of the practice. It is concluded that greater attention should be paid to the disease - which is as common as rubella, mumps or the more severe cases of chickenpox - in undergraduate teaching and in post-graduate research. The inadequacy of drug therapy highlights the importance of prophylaxis, and a greater public awareness of the potentially infectious nature of the common "cold sore" needs to be promoted.