Deoxythymidine kinase (TK) is an enzyme involved in DNA synthesis whose serum activity reflects the proliferative activity of tumors and correlates with prognosis in various malignancies. In ovarian cancer, the value of s-TK has not been studied so far. Therefore the serum levels of TK were investigated in patients with advanced ovarian cancer. Because considerable diurnal fluctuations of s-TK levels were reported previously, repeated determinations were performed over a 48-h time span. Fourteen patients (mean age +/- S.D., 56.1 +/- 8.0 years) with advanced ovarian cancer and five healthy volunteers (30.2 +/- 3.5 years) were studied. Serial determinations of s-TK and serum CA 125 (s-CA 125) levels were performed over a 48-h time period. S-TK and s-CA 125 were elevated (> 4.7 U/l and > 35 U/ml) in 10 patients at least once over the 48-h period, respectively. Linear regression analysis showed a strong correlation between s-TK and s-CA 125. However, three patients with consistently normal s-CA 125 values (< or = 35 U/ml) had elevated s-TK levels, indicating that these two parameters may be independent in some patients. Both s-TK and s-CA 125 levels showed considerable diurnal changes over the 24-h period in individual patients, in marked contrast to normal subjects. Individual peak-trough differences ranged from 0.1-8.5 U/l or 5-268% for s-TK, and from 4-75 U/ml or 15-100% for s-CA 125. Peak-trough differences of s-TK > or = 100% were found in five patients. The circadian fluctuations of s-TK and CA 125 did not show a regular circadian pattern nor any temporal covariation. This study demonstrates for the first time that s-TK levels may be elevated in ovarian cancer. In some patients, s-TK levels may exhibit considerable, irregular diurnal fluctuations. Repeated determinations should therefore be performed in situations where this marker is relevant for patient monitoring.