The frequency of HLA antigens was studied in 60 Israeli Jewish patients with ulcerative colitis (UC) and in 18 with Crohn's disease (CD), and it was compared to that in healthy, ethnically-matched controls. From this study it would appear that HLA-BW 35 is highly associated with UC in Israeli Jewish patients and that HLA-AW 24 is present at a high frequency in the "early onset" group and also in the group with "severe" disease, which was composed mainly of female patients. In the CD group, the frequency of the HLA antigens did not differ significantly from that in the control group.