STUDY OBJECTIVE--To assess the validity of diagnosis made in a general practice based morbidity recording from 1967-90. DESIGN--Clinical features of patients with a diagnosis of migraine headache and diabetes mellitus were compared with international diagnostic criteria for these conditions. For migraine headache the International Classification of Health Problems in Primary Care (ICHPPC) definition was used, while diabetes mellitus was defined according to World Health Organization (WHO) criteria. SETTING--The continuous morbidity registry of the Department of General Practice and Social Medicine, University of Nijmegen, has been recording data from four general practices (12,000 patients) continuously since 1967. The database is used for longitudinal clinical research. PATIENTS--All patients with migraine headache and living in the practice area at the time of study and matched controls with tension headache received a questionnaire asking about ICHPPC criteria symptoms of migraine. The medical records of all patients with diabetes mellitus at the time of diagnosis were compared with WHO criteria. MAIN RESULTS--In 85% of patients with migraine headache, the questionnaire confirmed the ICHPPC defined criteria. Twenty nine per cent of the matched controls reported migraine features. In 74% of the patients with diabetes mellitus the diagnosis was made in agreement with the WHO criteria: in 12% no clinical information from the time of diagnosis could be traced. CONCLUSIONS--The diagnoses of migraine headache and diabetes mellitus in the registry largely agreed with international criteria. The quality control of recorded data is satisfactory, and the registry might serve as a model for other primary care based databases.