OBJECTIVES The purpose of this study was to reveal parenchymal and vascular changes in acute prostatitis and to determine the role of color Doppler sonography in monitoring patients with this pathology. MATERIAL AND METHODS Twenty five patients with a clinical diagnosis of acute bacterial prostatitis (NIH 1) admitted to our institution were studied prospectively. Clinical, analytical and microbiological data were recorded. Color Doppler and transrectal ultrasonography (TRUS) were performed 1 week after antibiotic therapy and afterwards at 6 weeks, 3 and 6 month visits. The findings were recorded and scored using standardized criteria to characterize the degree and distribution of prostatic vascularity. RESULTS Blood flow was observed to the entire prostate capsule (grade 2) in 23 (92%) patients at first visit (1 week) and were present in 11 (44%), 6 (24%) and 2 (8%) at 6 weeks, 3 and 6 month visits respectively. The amount and distribution of blood flow within the prostatic parenchyma were evaluated using several criteria. Using the 2-point scale flow were classified as grade 2 22 (88%), 18 (72%), 12 (48%) and 3 (12%) patients at first, second, third and fourth visit respectively. Similar findings were noted using the Doppler spot scale which revealed that flow was grade 2 (15 spots or more) in 23 (92%), 19 (76%), 11 (44%) and 3 (12%) patients respectively. Mean number of Doppler spots in the prostate parenchyma was 23.1 +/- 11.1 at first visit, 10.3 +/- 9.5 after the end of therapy and 8.3 +/- 5.4 and 7.9 +/- 5.1 at 3 and 6 monthly respectively. CONCLUSIONS Patients with acute prostatitis require prolonged treatment and subsequent follow up for at least 6 months. Color Doppler sonography is a useful tool in monitoring response to treatment and in predicting clinical outcome.