Abstract Objectives. Patients with prostate cancer may have more of the complexed form of prostate-specific antigen (PSA) in the serum, whereas patients with benign prostatic hyperplasia have less of this complexed form and thus a higher proportion of the free form. However, the molecular basis for the lower percent of free PSA in patients with prostate cancer remains unknown, and considerable overlap in values exists. We examined this hypothesis in men with recurrent or persistent cancer after radical prostatectomy. These men, who have “pure” cancer in that they have no benign elements to their disease, should have very low percent free PSA values. Methods. Forty-six men with recurrent (persistent) cancer as manifested by rising PSA values (mean [±SD] 2.4 ± 2.5 ng/mL) after radical prostatectomy were available for analysis. Specimens were analyzed with the use of the Abbott AxSYM free and total PSA assays. The Mann-Whitney U test was used to compare percent free PSA values in this recurrent cancer group with values from a previously defined population of 413 men (225 with benign disease and 188 with prostate cancer before prostatectomy). Results. Median values of percent free PSA in the recurrent cancer group (8.4%) were significantly lower than values in the preoperative cancer (11.7%) or benign (17.4%) groups ( P <0.0001 for both comparisons). Among patients in the “pure” cancer group, 30 (65%) had values less than 10%; however, 4 patients (9%) had values from 15% to 19%, and another 4 (9%) had values of 20% or greater. Pathologically, patients with higher values (15% or greater) had aggressive disease. All patients with values of 20% or greater had evidence of seminal vesicle involvement or nodal disease. Conclusions. Although most cancers exhibit low values of percent free PSA, a significant proportion of aggressive tumors will demonstrate high values. Until this latter phenomenon can be explained, the widespread use of percent free PSA to distinguish benign from malignant disease or to stage confirmed malignant disease should be approached with caution.