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 < 0.05) less HSV infections, symptoms were significantly (p < 0.05) diminished in severity and healing time was significantly reduced (p < 0.05). L-Lysine appears to be an effective agent for reduction of occurrence, severity and healing time for recurrent HSV infection.