Evidence for an association between ABO blood groups and obsessive-compulsive neurosis has been reported (Rinieris et al. (1978]. Our study examines this hypothesis and the relationship between ABO phenotypes and personality traits in obsessive-compulsive neurosis. ABO blood types in 70 patients with obsessive-compulsive neurosis was determined and their distribution compared with that of a blood donor population. Premorbid personality was rated in 56 of the patient sample using a close relative as informant. This study failed to show an association between ABO phenotypes and obsessive-compulsive neurosis or premorbid personality traits. These results are discussed in the light of previous positive findings and in the context of the general criticisms of blood group-disease association studies.