The efficacy and tolerance of high dose intravitreal foscarnet for cytomegalovirus retinitis in patients with AIDS was studied. Foscarnet in a dose of 2400 micrograms was injected directly into the vitreous of 11 patients (15 eyes). Five patients had active retinitis (eight eyes, 53.3%), and received a 3 week induction therapy of six injections as the first step. Six patients had initial inactive retinitis (seven eyes, 46.7%), and received only maintenance therapy which consisted of a weekly injection. The main indications for intravitreal therapy were: myelosuppression, kidney toxicity, catheter related sepsis, or refusal of intravenous therapy. The patients were followed for a mean period of 16 weeks (range 8-28 weeks) and received a total of 304 injections. Vitreous foscarnet levels were measured by high performance liquid chromatography. After a 3 week course of induction therapy, complete resolution of the active retinitis was seen in 62.5% (5/8 cases), while 37.5% (3/8 cases) had partial resolution. No cases failed to respond or progress. The rate of relapse on maintenance therapy was 33% (five of 15 eyes) by 20 weeks, and two of these eyes did not respond to reinduction and progressed in involvement of the macula or optic nerve. Neither important local complications nor intraocular drug toxicity were observed. Vitreous foscarnet levels in two different patients were 896 mumol/l and 74.9 mumol/l at 22 3/4 hours and 42 1/2 hours after the injection. Intravitreal foscarnet appears to be a safe, effective, and useful alternative in patients with intolerance to intravenous and viral therapy.