QUESTIONS UNDER STUDY: To assess the prevalence of incidental synchronous primary cancers discovered by abdominal CT scan among prostate cancer patients. METHODS: Patients with prostate cancer in Geneva, Switzerland, were retrospectively analysed regarding incidental diagnosis of synchronous second primary malignancies, including a cohort of 398 patients treated from 1991 through 2001 with radical radiotherapy (RT) and a second cohort of 419 patients treated from 1991 through 2001 by radical prostatectomy (RP) in order to analyse the differences between RT and RP patients. Both cohorts were evaluated regarding incidence of synchronous second primary cancers, compared with that expected in the general population (Standardized Incidence Ratio, SIR). The influence of staging workup on the diagnosis of incidental primary malignancies was studied. RESULTS: Six synchronous cancers (4 renal, 1 pancreatic, 1 rectal) were observed on abdomino-pelvic CT-scan among 480 patients (398 RT patients and 82 RP patients) (1.2%), who had been subjected to staging workup. For renal-cell carcinomas (RCC) in 398 RT patients (RCC) SIR was 18.19 (CI [Confidence Interval] 4.96-46.57), (p <0.001). After exclusion of 12 patients from RP cohort (n:419) in whom the prostate cancer was an incidental finding during surgery for bladder cancer (SIR 33.50 [CI 17.83-57.28]), (p <0.001), 407 patients were observed. There was no synchronous RCC among 325 RP patients who had no CT-scan. CONCLUSIONS: In patients with prostate cancer, abdominopelvic CT staging detects incidental second primary cancers (mostly commonly RCC) with a greater frequency than that expected.