OBJECTIVE: To examine the changes inserum prostatic acid phosphatase (PAP) and two differentmolecular forms of serum prostate-specific antigen (PSA)in men treated with finasteride for benign prostatichyperplasia (BPH).METHODS: Eighty-five patients on finasteride therapy(5 mg/day) for symptomatic BPH comprised the studygroup (mean age 67 years, range 61-86). Blood sampleswere taken before and after 6 and 12 months of treatmentto determine PAP, total PSA (tPSA) and free PSA (fPSA).These prostatic markers were measured using the AxSYMPSA assay ®, AxSYM free PSA assay ® and IMx PAPassay ® systems (Abbott Laboratories Diagnostics, SouthPasadena, CA, USA).RESULTS: Although the mean total and free PSA levelsdecreased significantly, the mean f/t PSA ratio increasedonly slightly. The free PSA index (f/t PSA) remainedunchanged (over or under the 0.20 cut off point) during treatment in 95.3% of the patients. Serum PAP showed nosignificant variation.CONCLUSIONS: Serum PSA decreased to half after 6to 12 months of treatment; the rule "PSA x 2" is confirmedto be a valid reference for the interpretation of posttreatmentvalues. Serum PAP and f/t PSA ratios (0.20 cutoff point) remained practically unchanged and are thereforevalid tumor markers, without any modification, infinasteride-treated patients. This differential effect onserum PAP and PSA strongly suggests that expression ofthese proteins is regulated by androgens through differenttargets. We recommend the use of tPSA multiplied by 2 andf/tPSA without modification to discriminate between benignand malignant conditions in men treated with finasteride.