In a coded study, we treated 40 patients who had active herpes simplex corneal ulcers with either 1% trifluorothymidine (F3T) or 0.1% idoxuridine (IDU) drops; we treated 15 similarly afflicted patients, who had failed on IDU or vidarabine, with open 1% F3T drops. All dosages were at therapeutically recommended frequency. In the coded study there was no statistically significant difference between the drugs in rate of healing; mean initial ulcer size in both groups was approximately 7 mm2 and mean healing time was approximately 5.5 days. There was a significant difference, however, in the chances of successful healing; 96% of all F3T treated eyes and only 75% of IDU treated eyes healed completely within 14 days. In the open study, 87% of patients healed completely on F3T eyedrops. Although an insufficient number of patients were on concomitant coricosteroid therapy to provide statistical analysis, F3T-corticosteroid treated eyes (eight masked and open) all healed. The one IDU-corticosteroid treated eye in the masked study failed to heal.