We compared the frequency of severe ocular complications secondary to Herpes Zoster Ophthalmicus (HZO) in 232 patients. They were divided into three groups: 1) patients without treatment (n = 164); 2) patients treated adequately (n = 48) with acyclovir (ACV; 5 x 800 mg/d orally and ophthalmic ointment 5 x /d for a minimum of 7 days, given within three days after skin eruption); and, 3) patients treated inadequately (n = 20) with ACV (only topical treatment, insufficient doses, interrupted treatment, delayed treatment). Patients with no treatment or with inadequate treatments showed the same frequency of severe ocular complications (21% (34/164) and 25% (5/20), respectively). In contrast, when adequate treatment of ACV was given complications occurred in only 4% (2/48) of cases. This study emphasizes the need for prompt (within three days after skin eruption) and adequate (5 x 800 mg/d for at least 7 days) treatment of ACV to prevent the severe complications of HZO.