Radical prostatectomy is one of the most commonly used curative procedures for the treatment of localized prostate cancer. The probability that a patient will undergo additional cancer therapy after this procedure is largely unknown. The objective was to determine the likelihood of additional cancer therapy after radical prostatectomy. Data for this study were derived from a linked dataset that combined information from the Surveillance, Epidemiology, and End Results Program and Medicare hospital and physician claims. Records were included in this study if patient histories met the following criteria: ( a ) residing in Connecticut, Washington (Seattle-Puget Sound), or Georgia (Metropolitan Atlanta); ( b ) having been diagnosed with prostate cancer during the period from January 1, 1985, through December 31, 1991; ( c ) undergoing radical prostatectomy by December 31, 1992; and ( d ) having no evidence of other types of cancer. Patients were considered to have had additional cancer therapy if they had had radiation therapy, orchiectomy, and/or androgen-deprivation therapy by injection after radical prostatectomy. The interval between the initial treatment and any follow-up treatment was calculated from the date of radical prostatectomy to the 1st day of the follow-up cancer therapy. All presented probabilities are based on Kaplan—Meier estimates. The study population consisted of 3494 Medicare patients, 3173 of whom underwent radical prostatectomy within 3 months of prostate cancer diagnosis. Although radical prostatectomy is often reserved for localized cancer, less than 60% (1934) of patients whose records were included in this study had organ-confined disease, according to final surgical pathology. Overall, the 5-year cumulative incidence of having any additional cancer treatment after radical prostatectomy reached 34.9% (95% confidence interval [CI] = 31.5%–38.5%). For patients with pathologically organ-confined cancer, the 5-year cumulative incidence was 24.3% (95% CI = 20.0%–29.3%) overall and ranged from 15.6% (95% CI = 9.7%–24.5%) for well-differentiated cancer (Gleason scores 2–4) to 41.5% (95% CI = 27.9%–58.4%) for poorly differentiated cancer (Gleason scores 8–10). The corresponding figures for pathologically regional cancer were 22.7% (95% CI = 12.0%–40.5%) and 68.1% (95% CI = 58.7%–77.1%). Further treatment of prostate cancer was done in about one third of patients who had had a radical prostatectomy with curative intent and in about one quarter of patients who were found to have organ-confined disease. Given the common requirement for follow-up cancer treatments after radical prostatectomy and the uncertainties about the effectiveness of the various follow-up treatment strategies, further investigation of these treatments is warranted. [J Natl Cancer Inst 1996;88:166–73]