One hundred and twelve patients with acute infectious diarrhoea were entered in a double-blind randomised study in order to compare loperamide with a placebo. Of 82 evaluable patients, 38 received loperamide and 44 placebo for a maximum of 5 days. There were no significant differences in the number of loose stools during the first day of treatment, in the total number of tablets taken or in the total duration of the period of diarrhoea between the two treatment groups. The loperamide-treated patients had significantly fewer loose stools during the observation period of 5 days than did the placebo treated patients, median five vs. seven, a difference of little clinical importance. Excretion of bacterial pathogens was followed weekly in 13 of the loperamide treated patients (median 35.5 days) and in 18 of the placebo treated patients (median 22.5 days). This difference in the duration of excretion was not significant.