In a prospective, randomized, double-blind, placebo-controlled, cross-over study of forty-one patients we found that oral ingestion of 1,248 mg a day of L-Lysine monohydrochloride shows evidence of decreasing the recurrence rate of herpes simplex attacks in nonimmunocompromised hosts. A dose of 624 mg a day was not effective. L-Lysine may also be capable of decreasing the severity of symptoms associated with recurrences. Neither dosage showed any evidence of shortening the healing time compared to placebo.