The objective of this study was to compare the safety, tolerability and clinical response of once- versus twice-daily administration of didanosine given at a dosage of 270 mg/m2/day in children with symptomatic HIV-associated disease who were intolerant to or clinically deteriorated on zidovudine monotherapy. We carried out a randomized, open-label multicentre trial. Didanosine was supplied in buffered tablets, which could be chewed or dispersed in liquid. The children were recruited from 16 paediatric departments participating in the Italian Register for HIV Infection in Children. A total of 53 children (median age 5.5 years) started trial treatment; 26 were given didanosine twice daily and 27 once daily; 85% had AIDS and 98% had clinically deteriorated while on zidovudine therapy. Similar safety and tolerability results were demonstrated for the two schemes of therapy. A total of 11 children (20.7%) required discontinuation of didanosine for severe adverse events (five children (19.2%) in the twice-daily group; six children (22.2%) in the once-daily group, log-rank P = 0.81). Severe hepatic toxicity was uncommon (5.6%) while mild to moderate hepatic dysfunction was demonstrated in about 17% of the participants, without any difference between the two groups. Haematological toxicity was common (about 40% of the children, 11 in the twice- and 19 in the once-daily group) but never severe. Clinical pancreatitis and retinal lesions were never demonstrated. There was no significant difference in progression to death or to a new opportunistic infection between the two treatment regimens (log-rank P = 0.54). The modification of surrogate efficacy parameters during the study period was similar in the two groups. However, weight gain was poorer in children treated once daily. This study suggests that the safety and tolerability of 270 mg/m2/day of didanosine given once daily is substantially similar to that of the traditionally recommended schedule of two divided doses. Owing to the small sample and to the severity of the clinical condition of the children enrolled, no definite conclusions on the comparative efficacy of the two regimens can be drawn.