The accuracy of scrotal circumference as a predictor of specific pathologic changes within testicular parenchyma was assessed by scoring 121 Hereford and Simmental bulls for scrotal circumference, degree of germinal epithelial loss and semen quality. Scrotal circumference was not linearly related to the degree of germinal epithelial loss or to the percentage of tubules (Grade 4+ or higher, no germinal epithelium) for either breed (P>0.1). Thirty-two centimeters was the minimal acceptable scrotal circumference to ensure both a low prevalence of tubules with irreversible loss of germinal epithelium and acceptable semen quality. Scrotal circumference had a positive predictive value (the proportion of test-positive animals truly diseased) of 0.07 (95% confidence interval [CI] of 0 to 0.29) and a specificity (the proportion of test positive individuals that are diseased) of 0.89 (95% CI of 0.83 to 0.95) for 25% Grade 4+tubules. For acceptable semen morphology (>or=75% morphologically normal spermatozoa), scrotal circumference had a positive predictive value of 0.08 (95% CI of 0 to 0.34), a sensitivity (the proportion of diseased individuals testing positively) of 0.06 (95% CI of 0 to 0.24), and a specificity of 0.88 (95% CI of 0.80 to 0.95). For acceptable sperm motility (>or=30% progressively motile spermatozoa) scrotal circumference had a positive predictive value of 0.0 (95% CI of 0 to 0.22), a sensitivity of 0.0 (95% CI of 0 to 0.15), and a specificity of 0.87 (95% CI of 0.80 to 0.94). Although scrotal circumference is widely accepted as a useful test, its accuracy should be recognized as low when 32 cm is used as the minimal acceptable scrotal circumference in predicting unacceptable testicular histology or unacceptable semen quality in a healthy population of yearling bulls.