155 patients with 210 ovarian tumours were examined pre-operatively by CT from December 1984 to February 1988. 27 patients had dermoid cysts, unilateral in 25 cases and bilateral in 2 cases. These cystic teratomas were benign in 26 patients. In one case, malignant transformation occurred. CT detected 29/29 tumours (100%). Presence of fat (26/29, 90%), calcification (20/29, 69%), Rokitansky protuberance (25/29, 86%), tufts of hair (20/29, 69%), or fat fluid level (4/29, 14%), allowed to make a definite diagnosis in 28/29 cases (96%). These findings and especially demonstration of fat, appeared to be specific for dermoid cysts in our series. The diagnosis of torsion of the cyst associated with torsion of the adnexa was made in 2 patients. No abdomino-pelvic extension occurred in 26/26 patients presenting with a benign dermoid cyst and CT correctly diagnosed the lymph node and hepatic spread in the single case of cancer.