Urinary human chorionic gonadotrophin beta-core (hCG-betaC) was detected in 55-77% of gynecologic malignancies. The use of spot and early morning urine hCG-betaC as a tumor marker was explored with regard to the stability of the hCG-betaC level in serial spot urine samples collected within 24 hours and in early morning urine collected over 3 days. Thirteen patients with gynecologic malignancies were asked, before treatment, to collect serial urine samples voided within 24 hours. Nine of these 13 patients were also asked to save early morning urine for 2-3 consecutive days. Their urine was assayed for creatinine and hCG-betaC using an immunoradiometric assay. Variation of urine concentration was corrected by using the hCG-betaC/creatinine (betaC/Cr) ratio expressed in pmolg-1. Wide fluctuations of betaC/Cr ratios were found both in the serial spot urine within 24 hours and in early morning urine within 3 days. Eight per cent of the patients had one or more spot hCG-betaC level double or half the median of their own serial urine samples. Neither spot urine nor early morning urine hCG-betaC were suitable for use as a tumor marker for continuous monitoring because of the large fluctuation in hCG-betaC levels. The reason for such a wide fluctuation is not clear.