The pharmacodynamic relationship between a range of foscarnet exposure measurements obtained from studying nine patients receiving ongoing maintenance therapy for cytomegalovirus retinitis and a range of efficacy values (days to retinitis progression) obtained by independent examination of serial retinal photographs from the same nine patients was analyzed. In the resulting proportional hazards models, the foscarnet area under the concentration-time curve approached statistical significance (P = 0.11) as a predictor of decreased risk of retinitis progression.