Transrectal prostatic ultrasound is a minimally invasive, painless investigation that provides objective data on the topography of the prostate. Prostatitis is a condition difficult to diagnose in view of its varied symptomatology and lack of physical signs. The diagnosis currently rests on the finding of excessive numbers of leukocytes in the prostatic secretion obtained during the Stamey localization procedure. Seven ultrasound features having a significant correlation with a diagnosis of prostatitis have been identified: high-density and mid-range echoes, echo-lucent zones, capsular irregularity and thickening, ejaculatory duct echoes, and periurethral-zone irregularity. Ultrasonography confirms the focal nature and frequent peripheral-zone location of inflammatory prostate disease. Follow-up ultrasound studies over a short period showed that the echo-lucent zones may resolve to normal, mid-range, or high-density echoes in conjunction with a fall in the leukocyte count in expressed prostatic secretions. The ability to place a biopsy needle accurately under ultrasound control has enabled histologic assessment of the parenchymal features. The high-density echoes represent corpora amylacea; the mid-range echoes, inflammation, fibrosis, or both; and the echo-lucent zones, inflammation. The low specificity of mid-range and high-density echoes (51.9 and 40.7 per cent) and the low sensitivity (range 30.8 to 62.3 per cent) of the remaining five ultrasound features of prostatitis preclude identification of any one feature as being diagnostic of this condition. A possible exception may be the finding of echo-lucent zones, which may also prove useful in the monitoring of response to treatment. The detection of several signs within an individual's prostate suggests a diagnosis of chronic prostatitis, whereas a normal scan in a patient with symptoms of prostatitis is highly suggestive of prostatodynia.