Using log-linear Poisson modelling, trends in childhood cancer mortality among the population under 20 years of age in Spain are described over the 35-year period from 1956 to 1990. Overall cancer mortality and seven specific sites were considered: all leukaemias, Hodgkin's disease, non-Hodgkin's lymphomas, malignant brain tumours, kidney cancer, malignant bone neoplasms, and a broad category of ill-defined tumours. An age-period-cohort model was used to analyse the influence of age, period of death and birth cohort. Recent trends were estimated by restricting analysis to the last three 5-year periods. In general, mortality began to decline at the beginning of the 1970s, with reductions of 36% in males and 45% in females being registered between 1966-1970 and 1986-1990. The use of age-period-cohort models revealed an initially rising period effect attributable to diagnostic advances. The decline in mortality in post-1965 generations and the final downturn in the period effect are both most certainly a consequence of the remarkable progress achieved in the treatment of such tumours. During the final 15 years, there was a relative decline in mortality of approximately 20% every 5 years. However, in the case of malignant renal tumours in males and malignant bone tumours and non-Hodgkin's lymphomas in both sexes the situation remained stable.