The effect of loperamide was investigated in a double-blind, placebo-controlled study in 60 patients with irritable bowel syndrome (IBS). Active treatment was given in low dosage (4 mg nocte). The effect of treatment was assessed in clinical subgroups. In a group of patients with painless diarrhoea (n = 16) there was a highly significant improvement in stool frequency and consistency. In a group with alternating bowel habits and abdominal pain (n = 21) there was also a statistically significant improvement in stool frequency and consistency as well as significantly fewer painful days during loperamide treatment. Patients with alternating bowel habits and no pain (n = 12) experienced no symptomatic improvement, and patients with constipation (n = 9) generally felt worse on loperamide. No side effects were encountered. It is concluded that loperamide can be considered an alternative symptomatic treatment in some IBS patients whose main symptoms are painless diarrhoea or alternating bowel habits associated with abdominal pain.