During 1973 - 83 there was a pronounced rise in the number of new cases of herpes genitalis in Northern Ireland. This study of 127 patients with herpes genitalis shows that 30% had herpes simplex virus type 1 (HSV 1) and 70% had herpes simplex virus type 2 (HSV 2) infections. Of 41 sexual contacts who attended, only seven had herpes genitalis (caused in each case by HSV 2); another patient had a history of genital lesions, and one other of oral lesions, but HSV was not isolated from either patient. None of the other sexual contacts had a history of oral or genital infection. Of 31 women with herpes genitalis who consented to undergoing cervical cytology, five had atypical changes (all had primary herpes genitalis). Of these, four were infected with HSV 1 and one with HSV 2.