Serum levels of a cytokeratin 19 fragment, Cyfra 21–1, were measured using an immunoradiometric assay in 33 women before initial treatment and in 8 women with recurrent tumor. The serum level of Cyfra 21–1 was significantly increased in women with tumors of advanced stage (FIGO) and those with recurrence. The incidence of positivity for Cyfra 21–1 tended to increase with tumor spread. Compared with squamous cell carcinoma (SCC) antigen, the sensitivity of Cyfra 21–1 was comparable to that of SCC antigen. The serum level of Cyfra 21–1 and that of SCC antigen showed a positive correlation. While the use of the serum Cyfra 21–1 level may be limited in cervical cancer by its relatively low sensitivity, a combination assay of Cyfra 21–1 and SCC antigen may be useful in the diagnosis and follow-up of patients with cervical squamous cell carcinoma.