BOTH point mutations and structural aberrations of chromosomes are induced by ionising radiations, causing genetic variation and abnormalities in man and other organisms. The mutagenic effects are dose dependent and in Drosophila a linear relationship between dose and mutation rate has been shown for doses up to 5 R (ref. 1). Although man accumulates approximately 5 R of radiation from the environment in 30 yr of reproductive life, it is not known whether this is of any radio-biological consequence2. Nor is it known whether in man there is a threshold phenomenon at low doses (several hundred or thousand mr. per year), although there is greater repair of mutational or pre-mutational damage after low-dose irradiation3. In a coastal area of Kerala, South India, the background radiation is 1,500–3,000 mr. Yr−1 due to the presence of thorium-containing monazite mineral in the soil4–7 (Fig.1). A survey of the rat population in this area with respect to several measurable and non-measurable traits and of humans with regard to dermatoglyphics and demographic data such as fertility index, sex ratio and infant mortality rate revealed no mutational effects4,7,8. During an epidemiological study of nodular lesions of the thyroid in this area9, we noticed an apparently high prevalence of Down's syndrome and other forms of severe mental retardation10. We therefore made a house-to-house survey of developmental abnormalities in this area and in a comparable control area without high background radiation7 (Fig. 1). We also determined the frequency of chromosome aberrations in a sample of the normal population living in the study and control areas. The observations we report here support the view that radiation-induced genetic anomalies occur with above average frequency in the population living in the area with high background radiation. The area surveyed was the southernmost one-fifth of the Chavara-Neendakara strip (Fig. 1). In the thatched huts which constitute 75% of all households, the exposure risk is 1,500–3,000 mr. Yr−1, and personal exposure, as measured by calcium fluoride dosimeters, closely parallels the exposure risk in the households. The control area consisted of the Purakkade–Punnapura villages, with a background radiation of approximately 100 mr. yr−1 (ref. 7). Households were visited repeatedly to ensure examination of all members. Only gross abnormalities evident on clinical examination were recorded. Cytogenetic abnormalities were scored blind on slides prepared from 64-h micro-blood cultures11.