Our aim in this prospective study was to identify those patients who were found to have a colorectal cancer after a delay we considered unacceptable; this was taken as 6 months or more from initial presentation to a physician to diagnosis. It was then possible to determine the presenting complaints (always multiple) and the reasons for delay, in the hope that recommendations could be made regarding appropriate, rapid and thorough investigation of patients suspected of having a colorectal cancer. Of the total of 141 patients with colorectal cancer (108 elective, 33 emergency cases) under the care of one consultant during the period studied, 17 patients (12%) (10 men and 7 women), satisfied the criteria for late diagnosis. The mean age was 72.4 years (range 43-86 years). Five common presenting complaints were identified. They were, symptomatic iron deficiency anaemia, rectal bleeding, change in bowel habit, abdominal pain and weight loss. Incomplete imaging of the colon in patients with sinister presenting symptoms was the most commonly identified factor in delay of diagnosis. Inappropriate iron therapy and false-negative reporting of double contrast barium enema investigations were both seen in a number of cases. Other causes were, inappropriate surgical treatment and both clerical error and delay. The mean time for delay was 17.6 months (median 15 months). Late diagnosed cancers were most commonly found in the caecum and least commonly in the rectum. Colonic tumours of each Dukes' stage were identified, Dukes' B most common and Dukes' A least.