We studied the efficacy and safety of the topically acting steroid budesonide in an oral preparation for controlled ileal release in an open, uncontrolled trial. Twenty-one patients with active Crohn's disease localized to the distal ileum, ileocaecal region or ascending colon, entered the trial. The median age was 36 years and the median duration of Crohn's disease was 8 years. The patients received budesonide, in a controlled ileal release preparation, in a dose of 3 mg t.d.s. for 12 weeks, followed by a reduction to 2 mg t.d.s. for 6 weeks and finally to 1 mg t.d.s. for an additional 6 weeks. Primary variables were the modified Crohn's disease activity index (mCDAI), laboratory parameters and plasma cortisol levels. The mean mCDAI at entry was 268 (+/- 71 s.d.), dropping to 146 (+/- 91 s.d.) after 4 weeks of treatment and to 122 (+/- 87 s.d.) after a total of 12 weeks on 3 mg t.d.s. (P < 0.001). Following dose reduction, the mean mCDAI increased after 18 and 24 weeks of treatment. The erythrocyte sedimentation rate also fell significantly during the study period. Eighteen patients responded favourably during the first 12-week treatment period, and 13 completed the trial. Seven patients were withdrawn due to failure of treatment after reduction of dose, and four of those were treated surgically. No serious side-effects or significant corticosteroid-related side-effects occurred. The mean plasma cortisol levels decreased, but remained within normal range. Four patients were markedly suppressed on the highest dose of budesonide.(ABSTRACT TRUNCATED AT 250 WORDS)