OBJECTIVES Determine the cut point of free PSA rate for optimize the first prostate biopsy indication. MATERIAL AND METHODS Prospective trial between june 2002-september 2004, We included patients in first prostate biopsy with normal rectal digital examen, total PSA between 3-10 ng/ml and normal transrectal prostate ultrasound. We realize descriptive stadistic analisis of variables age, total PSA, prostate volume and % free PSA and analitic stadistic analisis with ROC curves of variables total PSA and % free PSA for determine as of her predicts the best one rate of prostate cancer. RESULTS We reclute 727 men with a mean age 62.91 years, total PSA mean 6.12 ng/ml, prostate volume mean 42.78 cc and % free PSA mean 15.22%. We had 106 prostate cancer, the prostate cancer rate in first biopsy was 14.6%; 77 cases had 1 lobe prostate cancer and 29 in 2 lobes. The most frecuent gleason was 6 (46 cases 43.4%) and the second gleason 7 (43 cases 40.6%). In the ROC curves analysis, total PSA had area under the curve 0.476 (p=0.3) and 0.611 (p=0.023) for % free PSA. The optime cut point for % free PSA in our trial was 19% (Sensibility 91.4% and Specificity 20%). The use of this cut point had allowed the saving us 138 biopsies (19.11%) with the lost one of diagnose of 10 cases of cancer of prostate. CONCLUSIONS The use of the % free PSA is useful and allows in our region the best indication of the patients who are going to first biopsy of prostate, avoiding the accomplishment of unnecessary biopsies.