Pharmacologic agents effective in the treatment of Crohn's disease confined to the small intestine are limited. The therapeutic efficacy of oral mesalazine in small bowel inflammation, although theoretically promising, remains unproven. In an open-labeled initial trial, timed-release 5-aminosalicylic acid (5-ASA), administered at a daily dosage of 30.6 +/- 9.0 mg/kg (mean +/- SEM) to children with active Crohn's disease involving the small intestine, was associated with improvement on the Harvey index in six of 12 patients treated for 8.1 +/- 3.9 weeks. In a subsequent prospective, double-blind study 14 children, ages 9.3 to 16.1 years, with active Crohn's disease limited radiologically in the small intestine were randomized to receive either timed-release 5-ASA [50 mg/kg/day (maximum 3 g/day)] or placebo for 8 weeks. Following a 4-week washout period, patients crossed over to receive the other study drug for a further 8 weeks. Six children completed the entire 20-week trial. The van Hees index improved among patients receiving 5-ASA for 8 weeks (delta = -18 +/- 6.4) but deteriorated among patients given placebo (delta = +14 +/- 4.1) (p < 0.05). Mean Crohn's Disease Activity Index (CDAI) decreased marginally after 8 weeks of 5-ASA treatment (delta = -48 +/- 38.2) but not with placebo (delta = -3.0 +/- 7.9) (p = 0.31). Of the eight noncompleters, more patients dropped out of the study because of lack of therapeutic response to placebo (n = 5) than to 5-ASA (n = 2).(ABSTRACT TRUNCATED AT 250 WORDS)