Prostatic volume was determined by transrectal ultrasonography before and after castration in 13 patients, and after radiotherapy in 24. Measurements were done after 1, 2 and 3 months, and subsequently at 3-month intervals. Significant volume reductions occurred in the castration and radiation groups within 3 months. The decrease in prostatic volume was significantly more pronounced in the castration group during the entire study (p less than or equal to 0.01). Patients with enlargement of the prostate predominantly owing to benign prostatic hypertrophy also had a decrease in volume. No increase in prostatic volume after initial reduction was encountered for up to 9 months. In several cases progression of metastases occurred with no increase in the volume of the primary tumor. Followup may be too short to encounter local recurrence after radiotherapy or hormone-independent growth after castration. Proctitis after radiotherapy created artifacts that probably led to inaccurate measurements with ultrasonography. The technique provides a new, accurate parameter for followup of conservatively treated prostatic cancer patients. The clinical importance of the technique still remains to be determined.