The effect of ovarian carcinoma on the circulating concentration of the cross-linked carboxyterminal telopeptide of type I collagen (ICTP) was studied in 17 patients before and during treatment. ICTP is a complex peptide released from type I collagen, which is the main organic component of bone matrix but also induced in ovarian carcinoma tissue and the peritoneal cavity by the tumour. CA 125 and the aminoterminal propeptide of type III procollagen (PIIINP) were also measured. The preoperative CA 125, PIIINP and ICTP concentrations were pathological in 16 (94%), nine (53%) and seven (41%) patients, respectively. There was a positive correlation between the PIIINP and ICTP levels in serum. After surgery and one course of cytotoxic chemotherapy, the median CA 125 concentration decreased whereas those of PIIINP and ICTP increased. During monitoring of the treatment response, patients with regressive disease showed lower CA 125 concentrations than those with stable or progressive disease. For PIIINP and ICTP there was no clear difference between these response categories, but altogether, simultaneously increasing serum CA 125, PIIINP and ICTP concentrations were consistently associated with poor prognosis.