The objective of this study was to evaluate the comparability of systemic aciclovir exposure at steady state in immunocompromised patients following oral valaciclovir 1000 mg tds and intravenous (iv) aciclovir 5 mg/kg tds. A two-centre, randomized, open label, two-way crossover study was undertaken. Patients aged 18-65 years who had undergone high-dose chemotherapy for cancer and were neutropenic (neutrophil count <0.5 x 109/mL) with normal renal function were recruited. The pharmacokinetic parameters of aciclovir after oral valaciclovir 1000 mg or iv aciclovir 5 mg/kg given as 1 h infusion, each administered every 8 h for seven doses, were compared. Fifteen patients were enrolled and 13 completed both treatments. The mean (s.d.) values for aciclovir after oral valaciclovir and iv aciclovir were: AUC0-8 76.3 (29.7) and 64.2 (20.0) microM x h; peak plasma concentration (Cmax) 26.6 (10.5) and 34.0 (11.9) microM; time to maximal plasma concentration (tmax) 2.01 (0.65) and 0.95 (0.19); and plasma elimination half-life (t1/2) 2.83 (0.91) and 2.44 (0.62) h, respectively. The mean absolute bioavailability of aciclovir from oral valaciclovir was 60 +/- 21%. Equivalent systemic exposure to aciclovir after oral valaciclovir 1000 mg and iv aciclovir 5 mg/kg was observed with AUC0-8 (oral/iv ratio = 1.16; 90% CI 0.98-1.39), whilst significantly reduced peak aciclovir concentrations were obtained with oral valaciclovir (ratio = 0.75; 90% CI 0.60-0.94). Oral valaciclovir offers a convenient, and possibly safer, alternative to iv aciclovir, delivering comparable systemic exposures with reduced peak levels. This may contribute to shorter hospitalization, reduced costs for healthcare providers and improved quality of life for patients.