The presence of early-onset sepsis significantly affects the prognosis of childhood acute lymphoblastic leukemia (ALL). We retrospectively analyzed all the septic episodes that occurred within 30 days of diagnosis of ALL from January 1978 to June 1994. Within the study period, 51 early septic episodes occurred in 45 patients. This accounted for 36.2% of the total number of septic episodes (141) that occurred during this period. The rate of early-onset sepsis in childhood ALL was 11.3% (45/397). The patients ages ranged from 11 months to 14.7 years. Fortyeight episodes of sepsis were caused by single organisms and three were polymicrobic. Pseudomanas aeruginosa was the most common isolate, followed closely by Staphylococcus aureus, Escherichia coli, Streptococcus spp and Salmonella spp. The case fatality rate of early-onset sepsis was 35.6% (16/45). The average time between diagnosis and death was 5.2 days (median, 1 d). Sepsis caused by P. aeruginosa had the highest mortality, with a case fatality rate of 72.7% (8/11), followed by polymicrobial infections (66.7%, 2/3) and candidemia (66.7%, 2/3). In the adolescent age group, initial white blood cell count < 2.0 x 10(9)/L, and depressed immunoglobulin level (especially IgA) were risk factors for a fatal outcome. During the study period, which covered 16.5 years, the incidence of Gram-positive cocci, especially Streptococcus spp, increased significantly and the overall outcome associated with early-onset sepsis improved.