Carcinoma is found unexpectedly in approximately 10% or more of the 400,000 prostatectomies performed annually in the United States. Patients with Stage A2 carcinoma die of their disease in only 35% of the cases. To alter the course of disease in these patients, 65% of Stage A2 patients may be treated unnecessarily by radical prostatectomy, radiation therapy, or hormonal therapy. An accurate method to predict the outcome of patients with Stage A2 carcinoma is needed. Histologic sections from 18 patients with Stage A2 prostatic carcinoma followed without further treatment until progression, or followed without progression, were evaluated by several investigators who did not have knowledge of patient outcomes and who employed standard pathologic grading systems as well as morphometric, cytophotometric, flow cytometric, and immunohistochemical techniques. Outcome was predicted correctly by random sampled absolute (17 of 18 cases) and relative (16 of 18) nuclear roundness factor (NRF), tumor volume expressed as percent of specimen (13 of 16), primary (13 of 18), secondary (14 of 18), sum (15 of 18), and worse (14 of 18) Gleason grades and prostate-specific antigen immunohistochemical findings (13 of 18) that produced statistically significant separation of the two groups. Significant separation was not obtained with Mostofi's pattern, nuclear, sum, and worse grades, Johns Hopkins' grade, absolute tumor volume, nuclear DNA content measured by image cytophotometric study of Feulgen-stained histologic sections and flow cytometric study of propidium iodide-labeled suspensions of nuclei obtained from paraffin blocks, nonrandom sampled NRF of worse and most prevalent neoplastic areas, and prostatic acid phosphatase and peanut agglutinin immunohistochemical study. NRF measured by a random technique best predicted outcome in these patients with A2 prostatic carcinoma and should be evaluated prospectively as a means for selecting patients who require therapy.