The prevalence and severity of irritable bowel symptoms was assessed by systematic questioning in 44 constipated volunteers, most of whom had documented slow intestinal transit. All but two had one or more of the following: passage of mucus, rectal dissatisfaction, bloating, and abdominal pain relieved by defecation. All the symptoms were more prevalent than in 17 normal volunteers or in 301 apparently healthy people studied previously. When 12 normal subjects were made constipated with loperamide all developed one or more irritable bowel symptoms. When 24 constipated subjects received effective laxative treatment the prevalence and severity of these symptoms fell markedly. The findings suggest that in some subjects the slowing down of intestinal transit is associated with irritable bowel symptoms.