Capsular transgression of prostatic carcinoma: evaluation with transrectal US with pathologic correlation. One hundred twenty-five patients with biopsy proved clinical stage A or B prostatic carcinoma were evaluated with biplane transrectal ultrasonography (US) prior to radical prostatectomy. Sonograms were evaluated for capsular transgression of the tumor into the posterior and posterolateral aspects of the glands as manifested by local contour deformity and irregularity or interruption of the periprostatic fat echoes. Correlation of the findings at US with the findings at pathologic examination of the step sections was obtained, and the presence and depth of capsular penetration were assessed. Of the 250 halves or hemispheres of the prostate gland that were evaluated, capsular penetration was seen at pathologic examination in 86. US enabled correct identification of pericapsular tumor spread in 59 of the 86 hemispheres but did not depict pericapsular tumor spread in 27 hemispheres. Absence of pericapsular tumor spread was verified at pathologic examination in 149 of the 164 hemispheres that either did not have tumor or did not show pericapsular tumor spread. Pericapsular tumor spread was incorrectly diagnosed in 15 hemispheres. A positive US diagnosis of pericapsular tumor spread correlated moderately well with the depth of penetration demonstrated at pathologic examination. Transrectal US is an effective noninvasive procedure that demonstrates the presence of prostatic cancer. 