Intestinal obstruction owing to colonic carcinoma is a relatively frequent cause of acute abdominal pain. The aim of this prospective study is to evaluate the prognostic factors that may influence the final outcome of those patients operated upon for an intestinal obstruction (OG) as opposed to those electively operated upon (EG). From September 1984 to March 1988, a total of 188 patients with colorectal cancer have been included in the study. One hundred thirty-five were EG, while 53 (28.1 percent) were OG. The mean ages were similar in both groups. Sex, morbidity, and mortality rates were equally distributed. Curative resection rate was significantly higher in the EG group (P = 0.029). Tumor staging tended to be significantly more advanced in OG patients (chi-square = 9.054; df = 3; P = 0.026). Multivariate analysis (proportional hazards model) showed that the only independent prognostic factor was tumor staging (P = 0.0000). Obstruction itself disappears as a predictive variable when tumor staging is introduced in the model. We conclude that obstructing colon carcinomas tend to be more locally advanced, that probably being the only reason for a worse long-term prognosis.