Among others, genetical factors have been discussed in the aetio-pathogenesis of inflammatory bowel diseases. This is supported by many studies reporting familial accumulation of Crohn's disease and ulcerative colitis. The use of HLA typing to demonstrate a genetical disposition has so far proved disappointing. Accordingly, if there is any association with definite HLA antigens, it is likely to be weak. We therefore investigated the HLA haplotypes in 13 families with multiple occurrences of Crohn's disease. Comparing observed and expected occurrences of HLA-haplotypes, we found a pronounced tendency towards common haplotypes in the affected siblings, uncles and nieces, and cousins. Although the data suggest a possible genetical disposition, additional factors such as microorganisms or possibly nutritional habits must be considered as causes for Crohn's disease.