The reliability of gray-scale ultrasonography in diagnosing torsion of the testicular appendages was studied in a series of 54 boys with acute non-traumatic scrotal pain. All boys were operated upon, and the appendages extirpated irrespective of their appearance at exploration; the final diagnosis was based on histological examination. Forty-two boys had appendicular torsion, 2 had testicular torsion and 10 had other diagnoses. Using the sign of appendicular torsion — an echogenic extratesticular structure situated between the head of the epididymis and the upper pole of the testis — as the criterion, 37 displayed a true positive, 9 a true negative, 3 a false positive and 5 a false negative diagnosis. Thus, the sensitivity was 88%, the specificity 75% and the positive predictive value 93% respectively. The echogenic mass varied in size from 3 to 17 mm in diameter. In 34 of the 42 cases of appendicular torsion extratesticular fluid was present, and 19 patients showed enlargement of the head of the epididymis. It is concluded that gray-scale sonography is an accurate and valuable tool in diagnosing torsion of the testicular appendages.