Faecal occult blood testing for 3 consecutive days is recommended for the detection of colorectal cancer. Is this adequate? Haemoccult tests were performed for 6 days on the faeces of 50 patients with colorectal cancer. Enteric-coated aspirin was given during the final 3 days to see if its systemic effect on coagulation would increase tumour bleeding. In 25 patients blood loss was quantified by radiochromium assay. Bleeding was slight and intermittent with large daily fluctuations in individual patients. Median daily loss before aspirin was 1.2 ml and after aspirin 2.5 ml. Thirty per cent of the tumours were Haemoccult negative using the standard 3-day regimen and 18 per cent were negative after aspirin. Over the 6-day period, 10 per cent were persistently negative. We conclude that new methods are required to improve the diagnostic yield from faecal occult blood tests.