Multiple primary cancers in the same individual occur rarely. Consequently, a large number of cancer patients have to be followed for long periods to obtain adequate information about their risk of subsequent tumour development. Studies of multiple malignancies are of interest, since they may provide information on common or opposite risk factors. In the present study, the risk of second primary cancers following carcinoma in situ of the uterine cervix diagnosed in Norway in the period 1970-1 992 was examined and quantified. Altogether, 37.00 I patients with carcinoma in situ were followed from the date of diagnosis until 31 December 1992. The follow-up period was divided into 5-year intervals. The results were expressed as standardized incidence ratios (SIR = O/E), and their 95% confidence intervals were given. A total of 1.037 second primary cancers in 989 individuals were identified. There was no overall excess of second primary cancers. However, there were differences depending on the site: cancers of the oesophagus, nose, nasal cavities, trachea, bronchus, lung, vulva, vagina, bladder and other urinary organs, and other skin cancers, excluding basal-cell carcinoma, occurred more frequently. A lower risk than expected was noted for cancer of the cervix uteri and cancer of the corpus uteri. There was a rising trend with time in the observed/expected ratio for cancer of urinary organs. In the group of patients evaluated, the likelihood of subsequent tumour development was no greater than in the general female population. Nevertheless, cancer sites of higher and of lower risk than expected were identified among the carcinoma-in situ patients.