Childhood leukaemia incidence in Costa Rica during 1981-96, among the highest in the world, was analysed by histology, gender, birth year, time period of diagnosis, age at diagnosis and region. Numbers of cases were extracted from the database of the National Cancer Registry (RNT) of Costa Rica. Person-years at risk were calculated from census data and post-census population estimates. During the follow-up, 918 cases of leukaemia in children under 15 years (510 boys, 408 girls) were reported to the RNT (41% of all childhood malignancies), with an overall age-standardised incidence rate of 56 per million person-years. Acute lymphocytic leukaemia (ALL) represented 79% and acute non-lymphocytic leukaemia (ANLL) 16% of the cases, with rates of 43 and 9 per million person-years respectively. There were downward trends in incidence of total leukaemias, ALL and ANLL and 'not otherwise specified' (NOS) combined. Incidence of ALL was highest at 1-4 years of age in boys and girls, whereas ANLL peaked in girls during the first year of life. During 1991-96, the decrease in ALL was significant (P = 0.042). A multivariable Poisson regression model identified significant excesses of ALL for boys, for age groups 1-4 and 5-9 years and for three out of seven regions. Possible reasons for the high rates in Costa Rica are discussed.