Abstract Background Evaluating the external validity of clinical trials requires knowledge not only of the study population but also of a relevant reference population. The main aim of this study was to present data from a large, contemporary, population-based cohort of patients with colonic cancer. Methods Data on patients diagnosed between 2007 and 2011 were extracted from the Swedish Colon Cancer Registry. The data, registered prospectively in a national population of almost 10 million, included over 99 per cent of all diagnosed adenocarcinomas of the colon. Results This analysis included 18 889 patients with 19 526 tumours (3·0 per cent had synchronous tumours). The sex distribution was fairly equal, and the median age was 74·1 (interquartile range 65–81) years. The overall and relative (cancer-specific) survival rates after 3 years were 62·7 and 71·4 per cent respectively. Some 88·0 per cent of the patients were operated on, and 83·8 per cent had tumours resected. Median blood loss during bowel resection was 200 (mean 311) ml, and the median operating time was 160 min; 5·6 per cent of the procedures were laparoscopic. Preoperative chemotherapy was administered to 2·1 per cent of patients; postoperative chemotherapy was planned in 90·1 per cent of fit patients aged less than 75 years with stage III disease. In patients operated on in an emergency setting (21·5 per cent), the preoperative evaluation was less extensive, the proportion of R0 resections was lower, and the outcomes were poorer, in both the short and long term. Conclusion These population-based data represent good-quality reference points.