A fast, simple, specific capillary electrophoretic method in the MEKC mode for the quantification of the antiviral drug ganciclovir is described. The separation was obtained using a 50 mu m id fused-silica capillary, 60 mM borax buffer (pH 9.25) containing 40 mM SDS using ethenoadenosine as the internal standard. Sample preparation was done by ultrafiltration with a Microcon 30000 kDa filter. The analytes were detected with UV detector at 254 nm. A sufficient sensitivity was achieved by using a bubble cell capillary. The linear range was from 0.5 to 10 mg/L with a LOQ of 0.5 mg/L. Correlation coefficients were better than 0.999 whereas inter- and intraday precision and accuracy were less than 10.7%. The analysis of patients' samples after administration of ganciclovir indicates that the method is suitable for drug monitoring in the clinic.