Incidence rates of malignant central nervous system (CNS) tumours in children in Costa Rica are presented in an international perspective. For the 16-year period 1981-96, a total of 256 CNS tumours were registered in children below age 15 years by the National Tumour Registry of Costa Rica. The age-standardised incidence rate was 15.2 per million person-years, with a male-to-female ratio of 1.4. The median age-standardised incidence rates of selected registries in other Latin American countries were 19.3, in other developing countries 12.0 and in industrialised countries 29.6 per million person-years. The comparatively low incidence rates in Costa Rica were evident in all diagnostic subgroups, most notably in the youngest age group and for tumours in the brain stem. In the Central Valley, where the capital and the only specialised paediatric hospital are situated, the crude incidence rate was 18.1 [95% CI 15.1, 21.1] compared with 10.5 [95% CI 8.3, 12.8] per million person-years in the rest of the country (RR = 1.7, 95% CI 1.3, 2.3). There was no evidence of any increase over time. The data in this study cannot exclude under-diagnosis and, to a lesser degree, under-registration as a partial explanation of the low incidence rates of malignant CNS tumours in children in Costa Rica.