This study reports the results of serial observations of simple haematological indices (haemoglobin concentration, white cell count and platelet count) in 25 patients who developed radiation bowel disease as a late complication of pelvic radiotherapy for malignancy. It is compared with a control group of 25 women patients who received pelvic radiotherapy but did not develop radiation bowel disease. There is a highly significant elevation in the platelet count (P less than 0.0001) at a time when the patients develop radiation bowel disease. The platelet count returns to normal after successful excision of the disease but the elevation recurs if further radiation disease develops in the urinary or gastrointestinal tracts. It is suggested that the platelet count may have a useful role in the diagnosis of radiation bowel disease.