Clinical, epidemiological and molecular data reported in the last 5 years have provided strong evidence that genetic factors are important in susceptibility to both Crohn's disease and ulcerative colitis. The model of disease inheritance which best fits the epidemiological data is that Crohn's disease and ulcerative colitis are related polygenic diseases. However, genetic heterogeneity is likely to be extensive, and may well underlie the variability in clinical presentation associated with inflammatory bowel disease. Considerable progress has already been made in identifying potential disease susceptibility loci using the technique of genome-wide scanning. There is optimism that international collaborative studies will allow fine mapping of the loci implicated on chromosomes 16 and 12. The identification of novel susceptibility genes has become a realistic goal for investigators. Once this has been achieved, there are likely to be immediate clinical benefits, both in understanding disease pathophysiology, and in disease management.