Erythema nodosum (EN) is a cutaneous reaction consisting of inflammatory, tender nodular lesions and is associated with a wide variety of disease processes. The aim of our study was to investigate the frequency of different aetiologies of EN. One hundred and thirty-two EN patients were investigated in a prospective study during the period 1984-1990. The evaluation of all patients began with a medical and family history and completed with a thorough physical examination and detailed laboratory and immunological work-up. In addition, various diagnostic procedures were performed where and when indicated. One hundred and ten patients (83%) were women. Their mean age was 41.0+/-14.0 years, range 18-79 years. In 35% the cause of EN was not found. Sarcoidosis was revealed in 28% of the patients, infections in 17.3% and tuberculosis in 1.5%. Other aetiologic factors were Adamantiadis-Behçet's syndrome (3.8%), pregnancy (6%), oral contraceptives (3.8%) and other drugs (3.8%). The aetiology of EN was not found in 35% of the patients. Sarcoidosis and infections were frequent causes of EN, whereas autoimmune rheumatic diseases rarely cause EN.