A double-blind, placebo-controlled, multicenter trial of oral L-lysine monohydrochloride for the prevention and treatment of recurrent herpes simplex (HSV) infection was conducted. The treatment group was given L-Lysine monohydrochloride tablets (1,000 mg L-lysine per dose) 3 times a day for 6 months. A total of 27 (6 male and 21 female) subjects on L-lysine and 25 (6 male and 19 female) subjects on placebo completed the trial. The L-lysine treatment group had an average of 2.4 (p less than 0.05) less HSV infections, symptoms were significantly (p less than 0.05) diminished in severity and healing time was significantly reduced (p less than 0.05). L-Lysine appears to be an effective agent for reduction of occurrence, severity and healing time for recurrent HSV infection.