We conducted a population-based cohort study in the province of Trieste, Italy, to assess whether the first-degree relatives of children with malignancies had an increased risk of cancer compared with the general population. We examined cancers occurring in all first-degree relatives of children who experienced malignancies under the age of 15 years between 1971 and 1993 (probands). A cohort of the 394 relatives of the 125 probands contributed 7,939 person-years of observation. Among the relatives as a whole, we found a statistically significant increased risk of developing all malignancies except non-melanoma skin carcinoma (21 observed relatives with cancer and 12.46 expected, for a standardized incidence ratio [SIR] of 1.69), of developing breast cancer (SIR = 3.09) and of developing haemolymphatic system neoplasms (SIR = 4.03). This was mainly due to the excess cancer risk in the relatives of probands with intracranial tumours, who showed a significant 3.1-fold risk for developing all cancers but non-melanoma skin tumours. Our findings and the previously reported steep rise in the incidence of childhood brain tumours in our area may imply that not only genetic factors but also shared environmental agents might be involved in the observed aggregation of cancer in the families of probands with intracranial tumours.