Squamous cell carcinoma arising in mature cystic teratoma of the ovary is rarely diagnosed preoperatively. We measured the pre-treatment serum levels of squamous cell carcinoma antigen in three stage I patients in order to evaluate its usefulness as a tumor marker for early detection of the disease. The serum levels were elevated in each case, to 26.9, 5.1, and 3.2 ng/mL, respectively, and rapidly dropped to within the normal range after treatment. All patients are recovered and show no evidence of recurrence, and the levels of serum squamous cell carcinoma antigen have remained within normal limits. These results suggest that measurement of the serum squamous cell carcinoma antigen level may be useful in diagnosing an early-stage primary squamous cell carcinoma arising in a mature cystic teratoma of the ovary. An elevated preoperative serum level of squamous cell carcinoma antigen may indicate the need for a rapid pathologic examination of the tumor to look for malignant elements and may influence the surgical regimen.