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.