Background: Phi is calculated from serum PSA, free/total (f/t) PSA and [-2]proPSA using the Beckman Coulter assay kit, and has been approved for use in patient selection for diagnostic prostate biopsy. We hypothesized that phi might also predict outcome of active surveillance. Methods: From 2002, we have done a prospective cohort study of active surveillance for men with T1/2, Gleason <= 3+4, PSA < 15ng/ml prostate cancer. Serum was banked at baseline. Monitoring included 6 monthly PSA and 2-yearly repeat biopsy. Treatment was indicated for PSA velocity > 1ng/ml/yr or Gleason >= 4+3 on repeat biopsy. We analyzed baseline phi with respect to time to treatment. A multivariate model was fitted using total PSA, PSA velocity, PSA density, Gleason score, % biopsy cores positive, T stage, and maximum % cancer in any biopsy core. The fit of this model was then compared with the addition of % f/t PSA and phi. Results: 370 patients were evaluable with a median follow-up of 5 years. The table shows the association between baseline phiand time to treatment. On multivariate analysis, the model with % f/t PSA was a significant improvement over base model (change in fit 41.1, p<0.001), and the model with % f/t PSA and phi was a significantly better fit than % f/t PSA alone (change in fit 11.1, p=0.001). Conclusions: In men with favorable risk prostate cancer, phi at diagnosis was a significant predictor of the outcome of active surveillance. The data require validation, but suggest that active surveillance is particularly attractive to men with a low phi.