The thymus-dependent immune system is involved in the disease process underlying Type 1 diabetes. Several studies have thus shown the distribution of peripheral T-lymphocyte subsets to be altered in diabetics. Unlike many other autoimmune diseases which improve during pregnancy, Type 1 diabetes has a higher incidence in pregnant than in non-pregnant women. Therefore, in this study the distribution of lymphocyte subpopulations in the blood was examined in pregnant, newly diagnosed Type 1 diabetic patients and compared with values from non-pregnant patients and healthy controls. The pregnant diabetics displayed a higher percentage of CD8+ cells (28.4 +/- 1.2%) than the non-pregnant diabetics (21.6 +/- 1.3%, p less than 0.005), but did not differ from the controls (29.0 +/- 1.3%). The CD3+ and CD4+ cell distribution displayed no significant difference within the groups. Among the pregnant diabetics a positive correlation was found between the percentage of CD4+ cells and the week of pregnancy when diabetes was diagnosed (p less than 0.01). No other correlations between immunological and clinical parameters were found. Thus the subsets of immune cells are not changed in an autoimmune direction for pregnant, newly diagnosed Type 1 diabetics as they are for non-pregnant. Therefore, the increased incidence of Type 1 diabetes in pregnant women is unlikely to be due to intensified autoimmune alterations in the immune system, but rather to changes induced in the beta-cells during pregnancy as discussed.