Most models to predict biochemical recurrence (BCR) of prostate cancer use pretreatment serum prostate-specific antigen (PSA), clinical stage and prostate biopsy Gleason grade. We investigated whether human glandular kallikrein 2 (hK2) and free prostate-specific antigen (fPSA) measured in pretreatment serum enhance prediction. We retrospectively measured total PSA (tPSA), fPSA and hK2 in preoperative serum samples from 461 men with localized prostate cancer treated with radical prostatectomy between 1999 and 2001. We developed a regression model to predict BCR using preoperative tPSA, clinical stage and biopsy Gleason grade. We then compared the predictive accuracy of this "base" model with a model with fPSA and hK2 as additional predictors. BCR was observed in 90 patients (20%), including 48 patients with a pretreatment tPSA <or = 14 ng/ml (13%), and 28 patients (10%) with a pretreatment tPSA <or = 10 ng/ml. Overall, the predictive accuracy of the base model (bootstrap-corrected concordance index of 0.813) was not improved after the addition of fPSA or hK2 (0.818). However, for men with moderate tPSA-elevation (tPSA <or = 10 ng/ml), addition of fPSA and hK2 data increased predictive accuracy (from a base model concordance index of 0.756-0.815, p = 0.005). The improvement in accuracy was not sensitive to the threshold for "moderately elevated" PSA. For patients with a moderate tPSA-elevation (tPSA <or = 10 ng/ml), which closely corresponds to concurrent disease demographics, BCR-prediction was enhanced when fPSA and hK2 were added to the conventional model. Measurements of fPSA and hK2 improve on our ability to counsel patients prior to treatment as to their risk of BCR.