Prostatic volume modifications were retrospectively analyzed by means of serial endorectal US in 50 patients affected with prostatic adenocarcinoma previously submitted to external radiotherapy. A progressive reduction in glandular volume (of low, medium, and high grade) was observed in 48/50 patients respectively within 3 months, between 3 and 6 months, and 6-9 months after treatment. After 9 months only lesser modifications were observed. A statistically significant correlation (p less than 0.05) was found between volume decrease percentage and histologic grading of the tumor, but not with its stage. Within the groups of patients affected with similar-grade neoplasms, a statistically significant difference (p less than 0.05) was observed, relative to the decrease rate of prostatic volume, between the patients with complete response and those with partial or no response. US monitoring of prostatic volume after radiotherapy, if correlated with histologic grading, can yield early predictive elements as to treatment outcome, thus contributing to select non-responsive patients to submit to biopsy and, if necessary, to therapeutic alternatives or combinations. Longer-term studies are needed to prove its effectiveness, to prognostic purposes, in the single patient.