Treatment of Crohn's disease (CD) in clinical remission is still a debated issue. Previous studies have shown a high risk of relapse for patients with CD in clinical remission (CDAI less than 150) but with some abnormally high laboratory parameters as well as a possible beneficial role of low-dosage steroid treatment in this group of patients. Furthermore, good results have been reported on the efficacy of 5-aminosalicylic acid (5-ASA) in moderately active CD. In our study we verified the efficacy of a slow-release oral 5-ASA preparation in preventing relapses in a group of patients in clinical remission but with raised laboratory parameters. Forty-four patients were randomized in a double-blind manner to receive either 5-ASA (2 g/day) or placebo for four months. Location of disease and previous steroid treatment were similar in both groups. One patient in the 5-ASA group discontinued the drug because of uterine bleeding. During the study period, 13 of 22 placebo-treated patients and 11 of 21 5-ASA-treated patients relapsed (corrected chi square = NS). Considering the location of disease, three of 10 patients in the 5-ASA group and six of nine patients in the placebo group with ileal CD relapsed (therapeutic gain with 5-ASA: 36.6%; 95% allowance for error from -6% to 79.2%). Moreover, in seven patients with ileal CD who remained in remission, we found a statistically significant decrease in alpha 1 acid glycoprotein and C-reactive protein from the second month of the study.(ABSTRACT TRUNCATED AT 250 WORDS)