The efficacy of a single intracutaneous BCG injection in prevention of recurrent herpes simplex (HS) was investigated in Tine-test-negative people. Most patients had a recurrence rate of HS of at least once per month. Between 1976 and 1983 109 patients were BCG-vaccinated (BCG sec Berna). Tine-test was negative in all subjects and converted to positive in 106 out of 109 patients after vaccination. Patients suffered from HS for many years and thus acted as their own controls comparing them before and after BCG. After vaccination all patients remained herpes-free for at least 4-6 months. In a separate study a follow-up up to 10 years was done. This showed a decreasing effect of BCG over the years with still 21 (19%) completely herpes-free patients after 3 years. 10 patients (9%) even had been free of herpes for more than 6 years. Reduction of frequency and duration of HS after BCG was statistically significant (p less than 0.01). Since the BCG vaccination can be assumed to be virtually without risk, a Tine-test followed by BCG vaccination should be considered as treatment of choice in patients with recurrent HS for prevention of recurrences.