Investigate the causes and characteristics features of erythema multiform at The National Hospital of Dermatology and Venereology. Objectives: To investigate the features of erythema multiform associated with herpes simplex virus or drugs at the National Hospital of Dermatology and venereology (NHDV). Materials and Methods: a prospective study basing on 41 patients with erythema multiform. PCR with HSV and drugs reaction tests were realised. Results: 8/41patients (19,5 percent) were HSV(+),in which 17,1 percent of patients suffering major type, 2,4 percent with minor type. There was 9/41 patients (22 percent) related with drugs. In which 14,6 percent present major and 7,3 percent with minor type (p 0,05; Fisher' exact test). The reoccurence was 26,8 percent of patients., The reoccurence of patients asociated with HSV (13,5 percent) was nearly trifoled to the drugs group (4,9) percent; 12,2 percent of patients with HSV(+) present cutaneous manifestations more emerge from normal skin area than ttiat of group with drug reaction. There was not difference between 2 groups about pre-syndrome. Conclusions: one fifth of patients associated with HSV, 22 percent of patients related to drugs. The patient with HSV+ suffering the minor type and most of drug-related patients was major type. The lesion in patients with HSV(+) was more emerged than that related to drugs.