OBJECTIVES: I. Determine the acute and late grade 3-5 genitourinary (GU) and gastrointestinal (GI) toxic effects of external beam radiotherapy followed by permanent source brachytherapy using interstitial iodine I 125 seeds in patients with intermediate risk, clinically localized adenocarcinoma of the prostate. II. Determine the freedom from prostate specific antigen (PSA) failure, overall survival, disease specific survival, and local and/or distant clinical relapse rates in patients treated with this regimen.