The possible relationship between Lewis red cell groups and secretor status and diabetes mellitus has been investigated in diabetic patients from Northwestern Ethiopia. The Lewis negative phenotype [Le(a-b-)] showed similar frequencies in diabetics and a control sample. Determination of the secretor status revealed a tendency to higher non-secretor rates in diabetics, particularly of the insulin-dependent type, in comparison with non-diabetic Ethiopians. A lack of effective immune protection from secretory IgA antibodies as a plausible explanation for the relationship between non-secretor status and IDDM is discussed. However, the available data from our study showed no statistically significant association between secretor state and IDDM. Without a detailed genetic characterization of our diabetic patients (HLA association data) it will be difficult to define precisely the postulated contribution of the Se gene to the aetiopathogenesis of IDDM.