To determine the prevalence of the in vitro resistance rate of cytomegalovirus (CMV) to gancyclovir (GCV) and foscarnet (FOS) among patients with AIDS and chorioretinitis. Furthermore, in vitro sensitivity results were compared with the clinical response to therapy.Thirty-six patients with the diagnosis of CMV retinitis and AIDS were included in the study. Antiviral sensitivity testing was performed to 51 clinical CMV strains from these patients. The in vitro sensitivity was compared with the clinical response to therapy. The resistance criteria were the inhibitory dose50 (ID50) GCV > 5 M and ID50 FOS > 400 M.None of the CMV tested strains was resistant to GCV or FOS; however, six patients who had relapses of chorioretinitis while on maintenance therapy and isolation during the condition had mean ID50 of strains (n=8) of 1,95 M for GCV (standard deviation [SD] of 0.71) and 115.2 for FOS (SD, 34.7). These patients responded well when drugs were used at induction doses.The in vitro sensitivity testing is not a good predictor of infection control when the drug is used at maintenance doses. This suggests that these strains should be classified in the intermediate resistant category.