In response to public concern about an increase in the incidence of leukemia among children in southwestern Sardinia (Italy), incident cases of childhood cancer (ages 0-14) were ascertained among residents in the province of Cagliari, which comprises all of southern Sardinia, in 1974-89. Completeness of the ascertainment of leukemia cases was validated by comparison with estimates derived from official statistics of mortality and survival curves. A significant excess risk of childhood acute lymphoblastic leukemia (cALL) was found for children residing in the town of Carbonia. The risk was highest in 1983-85, when seven cases occurred versus 0.8 expected. No birth-cohort effect was observed. The cALL incidence rate was significantly higher among children born and residing in Carbonia than among children born in Carbonia but residing elsewhere. However, the cALL cases did not cluster within the town of Carbonia. The proximity of the largest industrial settlement in the region of Sardinia raised the suspicion that environmental pollution was responsible for the observed excess. Information about industrial emissions from this settlement prior to the appearance of the cALL cluster was not sufficient to reject or confirm the hypothesis.