Age-related differences in the incidence and immunological subtypes of acute lymphoblastic leukaemia (ALL) suggest that it may be composed of more than one disease entity, each with different aetiologies. Childhood leukaemia (of which the majority of cases are ALL) has been suspected of having an infectious aetiology, but few studies have systematically examined ALL for clustering by age group. The aim of this study was to examine ALL for evidence of space-time clustering of date and place of diagnosis by age group. Knox space-time analysis was carried out separately for three different age groups: childhood (0-14 years), young adult (15-34 years) and older adults (35-79 years). Data on 968 cases of ALL aged 0-79 years, arising during 1984-1993 in the areas covered by a specialist population based register of leukaemias and lymphomas in parts of the U.K., were used in the analysis. Space-time clustering of diagnoses was limited to children aged 0-14 years. It was more prominent in those diagnosed in the period 1984-1988, than in those diagnosed in 1989-1993. The clustering may indicate an infectious aetiology for childhood ALL, or could be the result of episodic exposures to some environmental hazard.