Prostate cancer is histologically heterogeneous as reflected in the 5 patterns of the Gleason grading system. Gleason grade correlates with volume, extent and prognosis. Serum prostate specific antigen (PSA) levels also correlate with tumor volume but the degree to which grade correlates with PSA has not been precisely defined. To quantify this relationship further, we prepared maps of each grade of cancer in 86 radical prostatectomy specimens from patients with clinical stage T2 cancer. The median per cent of the volume of cancer per prostate composed of grade 1 was 0%, while it was 1% for grade 2, 84% for grade 3, 5% for grade 4 and 0% for grade 5. We stained 95 cancer foci (grades 1 to 5) in 40 of these specimens for PSA. The presence and intensity (0 to 3+) of staining in more than 33,000 acini (or cells) correlated inversely with grade (p < 0.0001). Nearly all acini in grade 1 and most in grade 2 stained positive (2 to 3+) for PSA; 87% were positive but with less intensity in grade 3. While many grade 4 (79%) and grade 5 (49%) cells were positive, the intensity of staining was weak. Serum PSA levels correlated with total tumor volume (r = 0.67) but serum PSA levels per cm.3 of cancer decreased with increasing grade (r = -0.24 and p < 0.02). These studies confirm the strong inverse correlation between Gleason grade and the PSA content of prostate cancer. Since more than 85% of grade 3 acini stained for PSA and grade 3 made up the largest portion (84%) of cancer, the predominant contributor to serum PSA levels from prostate cancer was Gleason grade 3. The other grades contribute relatively little to the serum PSA levels either because of the small volume (grades 1 and 2) or the diminished PSA content (grades 4 and 5).