We studied the alteration in the clinical features of herpes simplex virus-induced keratitis (HSK) after the introduction of acyclovir (ACV) (in 1986). Over a nine-year period, this study investigated 101 patients (52 men and 49 women) whose conditions were diagnosed clinically as HSK. In addition, HSK recurred in 48 (47.5%) of the 101 patients. The distribution of clinical types of primary HSK before 1985 did not differ from that after 1986. Recurrent HSK was significantly more frequent in patients aged 26 to 40 years (75.0%). Although the five-year recurrence rates in primary HSK treated with ACV were significantly lower than those in patients treated with idoxuridine, the recurrence frequency before 1985 was equal to that after 1986. The discordance of the decreased recurrence rate in ACV-treated patients and the high recurrence frequency in patients with recurrent disease after the introduction of ACV were considered to be the result of the incomplete inhibitory effect of this drug.