An infant cohort treated for skin haemangioma with 226Ra between 1930 and 1965 (n = 11,807) was studied. The median age at treatment was 5-months and 88% were treated before 12 months of age. This cohort was followed up in the Swedish Cancer Registry during the years 1958 to 1989, giving 370,517 person-years at risk. A total number of 248 malignancies have been observed and the standardized incidence ratio (SIR) was 1.21 (confidence interval (CI) 95%, 1.06-1.37). Significantly increased numbers of cancers were found in the central nervous system, 34 cases (SIR = 1.85, CI 95% 1.28-2.59), the thyroid, 15 cases (SIR = 1.88, CI 95% 1.05-3.09) and other endocrine glands, 23 cases (SIR = 2.58, CI 95% 1.64-3.87). The absorbed dose in 11 specified risk organs has been estimated using a phantom of the size of a 5-6-month-old child. The mean absorbed dose in the thyroid was 0.12 Gy and the excess relative risk (ERR) for thyroid cancer was 7.5 per Gy (CI 95% 0.4-18.1). The mean dose in the central nervous system was 0.077 Gy and the ERR for brain tumours was 10.9 per Gy (CI 95% 3.7-20.5). This cohort gives a unique opportunity to analyse long-term effects of low-dose irradiation during infancy.