In a randomized open trial foscarnet 90 mg/kg b.i.d. 5 days for 3 weeks was compared to 90 mg/kg b.i.d. daily in severe gastrointestinal cytomegalovirus disease in HIV-infected patients. Thirty-eight patients were randomized, 36 were evaluable (all male, age 24-54 years, median 40 years; CD4/microliter 0-150, median 10). Treatment efficacy was evaluated based on a score consisting of symptoms, endoscopic and histologic examination. In the 5-day treatment group 10/16 (62%) patients responded to treatment, in the 7-day treatment group 13/20 (65%), with symptoms resolving in most patients after 1 week. Side effects and adverse events were seen in 13 patients in the 5-day treatment group and in 15 patients in the 7-day treatment group. Laboratory abnormalities were common in both groups, in one patient reversible renal insufficiency developed. Efficacy and safety of treatment 5 days a week was comparable to the standard regimen.