OBJECTIVES The aim of this study was to compare the tolerance and effects of lactulose taken at a daily dose of 20 mg in one or two intakes, in the treatment of idiopathic chronic constipation. METHODS A total of 121 outpatients (30 men, 91 women, mean age 55) was included in a prospective double blind crossover multicentric study. Patients took lactulose according to two different ways of administration and for two successive 3-week periods. Each period consisted in 2 weeks of adaptation and one week of evaluation. At the end of each period, a global visual satisfactory scale was evaluated as a global index. Patients collected every day the frequency and the aspects of the stools, the quality of exoneration and the existence of digestive symptoms. RESULTS Number of stools per week (7.29 +/- 3.20 vs 7.08 +/- 3.03), consistency, quality of exoneration, were equivalent whatever the number of intakes and the period of treatment. With one or two intakes, the satisfactory index was the same but significantly better at the end of the second period. With one or two intakes, frequency and intensity of side effects were low and identical. They mostly occurred during the first period. CONCLUSION Effects and tolerance with one single intake of lactulose (20 g) in the evening and with two intakes, were found to be equivalent. Effect on exoneration persists during the six weeks of administration and then tolerance improves.