This prospective study investigated the value of segmental colonic transit time studies in the surgical treatment of slow transit constipation. Overall, 346 patients with chronic constipation were analysed; slow transit constipation was diagnosed in 86 (25 per cent). Based on the results of segmental colonic transit time studies, 18 patients underwent partial left-sided colectomy and 24 subtotal colectomy. Recurrent constipation was seen in three of 18 patients and seven of 24 respectively. Severe abdominal discomfort was noted in six of 18 and 15 of 24 respectively. Disabling diarrhoea and faecal incontinence developed in two of 14 and five of 20 patients with an anastomosis respectively. Although these results indicate that segmental colonic transit time studies are useful in selecting patients with slow transit constipation for partial left-sided or subtotal colectomy, both procedures should be performed with prudence.