Objectives. Finasteride is known to lower total serum prostate-specific antigen (PSA) levels by approximately 50%. Terazosin is thought to have little or no effect on serum PSA concentration. The objective of our study was to determine the effect of finasteride and terazosin on serum total and serum free PSA levels and the ratio of free to total PSA. Methods. We identified 69 men with symptomatic benign prostatic hyperplasia (BPH) who had been receiving 5 mg/day (n = 33) of finasteride or 2 to 5 mg/day (n = 14) of terazosin or no therapy ('watchful waiting') (n = 22). The three groups were compared with respect to pretreatment total serum PSA levels and post- treatment total, free, and percent free serum PSA levels. Results. Median (± semi-interquartile range [SIR]) pretreatment total serum PSA levels (ng/mL) were not significantly different in men taking finasteride (2.8 ± 1.9), terazosin (2.2 ± 2.5), or undergoing watchful waiting (5.5 ± 1.4) (P = 0. 1 2). The median (±SIR) post-treatment total serum PSA levels (ng/mL) were significantly lower in the finasteride group (1.1 ± 1) when compared with the terazosin (2.5 ± 1.5) or watchful waiting (4.3 ± 2.8) groups (P = 0.016). Only the finasteride group had significantly lower post-treatment total serum PSA levels compared with pretreatment levels. The median (±SIR) post-treatment free PSA levels were significantly lower in the finasteride group (0.26 ± 0. 16) compared with the terazosin (0.54 ± 0.5) and watchful waiting (0.85 ± 0.5) groups (P = 0.0015). However, the median (±SIR) percent free PSA was not significantly different in the finasteride (225 ± 6), terazosin (22 ± 4), and watchful waiting (25 ± 5) groups (P = 0.66). Conclusions. Finasteride appears to lower total and free PSA levels equally in men with BPH and does not appear to change the ratio of free to total serum PSA, Terazosin does not appear to alter total or free serum PSA levels in men with BPH. The percent free PSA could potentially be used to screen for prostate cancer in men taking finasteride. Prospective studies are needed to further evaluate this issue.