Purpose Quality of life in colorectal cancer patients may be affected by colostomy and treatment, but relevant studies are still scarce and contradictory. The present study aimed to evaluate the association between colostomy time and treatment type with quality of life in colorectal cancer patients. Methods A prospective observational study of 41 patients with colorectal cancer was conducted on three occasions T0, T1 and T2 (0–2; 3–5 and 6–8 months after ostomy surgery, respectively). The treatments prescribed were: surgery alone, chemotherapy or radiotherapy, or chemoradiotherapy. European Organization for Research and Treatment of Cancer questionnaires were used to evaluate quality of life. Worsening clinical changes were evaluated considering difference in scores between times of surgery ≥±9 points. Results Regarding ostomy surgery, scores in physical function improved between T0 and T1 and these better scores were maintained at T1 to T2. The same was observed for urinary frequency, appetite loss and dry mouth. Chemoradiotherapy was associated with worse scores for global health status, nausea and vomiting, bloating and dry mouth. Although significant differences were not observed in some domains in the Generalized Estimating Equations analysis, patients showed noticeable changes for the worse in the pain, anxiety, weight concern, flatulence and embarrassment domains during these periods. Conclusions Colostomy improved quality of life at 3–5 months in most domains of quality of life and remained better at 6–8 months after surgery. Chemoradiotherapy had a late negative influence on quality of life. Health teams could use these results to reassure patients that this procedure will improve their quality of life in many functional and symptomatic aspects.