Colorectal leiomyosarcoma (CLM) is an uncommon tumour. Reports of its occurrence have been published mostly as single cases or small series. This study documents 12 cases of CLM that were seen over a 28-year period in Saskatchewan. The annual incidence of CLM was 0.45 per million people and constituted 0.12% of all colorectal malignant tumours seen during the study period. CLMs had a predilection for the rectum and sigmoid and commonly were associated with rectal bleeding or abdominal pain. More than half the tumours were detected by sigmoidoscopy. A correct preoperative or intraoperative histologic diagnosis of leiomyosarcoma was made in only two out of six cases. A potentially curative surgical procedure was done in 10 of the 12 patients. The mean follow-up was 6.9 years. Eight patients had tumour recurrence or metastasis, or both. From the findings of this study the authors recommend wide excision of colorectal smooth-muscle tumours whenever there is a suggestion of malignancy.