Erythema multiforme may be recurrent and the oral cavity is often affected. A series of 82 patients with unequivocal recurrent cutaneous erythema multiforme were examined to determine the incidence and nature of oral lesions. Seventy per cent of patients had oral lesions, comprising multiple, large, shallow, extremely painful and debilitating ulcers, which affected the entire oral mucosa in over 20%. The buccal mucosa and tongue were the most frequently affected sites in the remainder and the lips were affected in 13% of patients. Lesions generally lasted for 1-3 weeks. In over 60% of cases these attacks followed an episode of herpes simplex virus infection during the preceding fortnight. Recurrent attacks showed a different site distribution from the initial attacks, with a greater proportion having genital as well as skin and oral mucosal involvement. Detailed case histories of five patients are presented to illustrate the role of azathioprine in treating oral lesions and to document a familial case of recurrent erythema multiforme, with severe mucosal involvement.