SUMMARY ABO and Rhesus blood group data have been examined for 1,680 patients treated for gastric cancer in four London hospitals.The risk of developing gastric cancer was estimated to be 16% higher for group A subjects than for group 0 subjects, in close agreement with many previous reports throughout the world. More detailed analysis failed to support the suggestion that there was any special correlation between either ABO or Rhesus blood groups and the site of origin of the tumour within the stomach or the sex or age of the patient.Male predominance among the patients was noted to be greater at ages 40-69 years than at other ages and greater for tumours of the upper two thirds of the stomach than for tumours that arose distally.The relevance of genetic factors to the aetiology of gastric carcinoma is widely accepted and is derived from familial and blood group studies (McConnell, 1966). The precise contribution of these genetic factors, however, is unknown. Aird, Bentall, and Roberts (1953) were the first to study ABO blood groups in a large number of patients with gastric cancer and found an excess of group A and a deficiency of group 0 in comparison with control patients. Subsequently, at least 71 controlled series have been published; in 55 of these the cancer patients showed an excess of group A, in 14 a slight predominance of group A, and in only two a deficiency of group A (McConnell, 1966). These world-wide studies show that group A increases the risk of gastric malignancy by about 20 %, and that this proportional increase is approximately the same irrespective of whether the overall incidence is low, intermediate, or high.To define more exactly the contribution of this genetic effect to the pathogenesis of gastric cancer, workers have examined blood groups in relation to such variables as tumour site, histology, gastric acidity, and age and sex of the patient. Among these studies was that of Doll, Swynnerton, and Newell (1960) who found a decreasing ratio of group A to