A novel, nonscrotal, transdermal delivery system for testosterone therapy has been marketed for treatment of hypogonadal men. The usual dose of this system is two 2.5 mg/day systems applied daily. A new system has been developed that administers a dose of 5 mg/day using a single patch rather than two patches. A randomized, steady-state, four-period, replicate-design, open-label, crossover study was conducted to assess the bioequivalence of the two testosterone transdermal delivery systems in postpubertal, hypogonadal men: two 2.5 mg/day patches as the reference regimen (R) and one 5 mg/day patch as the test regimen (T). 21 men were enrolled, and 20 completed the study. Each subject was randomly assigned to one of four sequences (R1-R2-T1-T2, T1-T2-R1-R2, R1-T1-T2-R2, T1-R1-R2-T2), such that each subject received each regimen during two study sessions. Two subjects were inadvertently treated according to the sequence T1-R1-T2-R2. Patches were applied to the upper arm, thigh, and back in the evening on days 1, 2, and 3, respectively, of each study session. Serial blood samples were obtained for pharmacokinetic analysis of testosterone for 24 hours after patch application on day 3 of each study session. The two formulations would be considered bioequivalent if the 90% confidence intervals (CI) for the ratios of the adjusted geometric means for T:R for both area under the concentration--time curve from 0 to 24 hours (AUC0-24) and maximum concentration (Cmax) were completely contained in the interval (0.80, 1.25). Mean values for AUC0-24 and Cmax were similar for the two formulations. The T and R formulations were found to be bioequivalent based on both AUC0-24 (90% CI 0.96, 1.08) and Cmax (90% CI 0.92, 1.07). The median time to Cmax was also similar, indicating comparable rates of testosterone absorption for both formulations. Based on this analysis, the testosterone transdermal system 5 mg/day patch is bioequivalent to two of the 2.5 mg/day patches. Both systems were safe and well tolerated in hypogonadal men.