A study has been made of the relationship between socio-economic factors and the incidence of acute lymphoblastic leukaemia (ALL) of childhood. It was found that the incidence of childhood ALL in 12 areas of Queensland. Australia, correlated well with some indicators of above-average socio-economic status for these areas. A similar result was found when Brisbane City was studied separately. Social class was determined from the fathers' occupations at the time of diagnosis. There was found to be a higher than expected number of ALL cases in each of the upper 5 social classes and a lower than expected number in the remaining 2 lower classes. Factors associated with differences in lifestyle amongst the various social classes may increase or decrease the risk of development of ALL.