There are limited data regarding the pathological features of incidentally detected prostate cancer. Examination of cystoprostatectomy specimens obtained during bladder cancer treatment affords a unique opportunity to examine incidentally detected prostate cancer and determine its relationship with clinically detected prostate cancer obtained during radical prostatectomy. We compared the pathological findings of incidentally detected prostate cancer in 132 consecutive cystoprostatectomy specimens from patients treated for bladder cancer with a consecutive series of 228 radical prostatectomy specimens from patients treated for prostate cancer. All specimens were totally embedded and whole-mounted. Karyometry was evaluated in select subsets of patients from the study groups. Incidentally detected cancer was found in 42% of cystoprostatectomy specimens, and the cancers were of lower Gleason score and lower pathological stage with fewer positive surgical margins than in clinically detected cancers in age-matched radical prostatectomies. High-grade prostatic intraepithelial neoplasia (PIN) was present in 82% of radical prostatectomy specimens, in 70% of cystoprostatectomies with incidentally detected prostate cancer, and in 54% of cystoprostatectomies without prostate cancer. Mean nuclear and nucleolar area was lower in incidentally detected cancer and PIN when compared with clinically detected cancer and PIN, respectively, similar to the results with proliferative indices. We conclude that incidentally detected cancer is less aggressive than clinically detected cancer.