Fifteen women with positive islet cell antibodies were identified in a group of 115 consecutive patients found to have impaired glucose tolerance in pregnancy. These subjects were postulated to be at increased risk of later developing type 1 diabetes mellitus. They were examined post--partum for HLA types known to be associated with this disease and for any increase in Interleukin 2 receptor expression or alteration of T cell subsets of possible relevance to its pathogenesis. Fifteen women negative for islet antibodies and with normal glucose tolerance during previous pregnancy and 15 women with a normal fasting plasma glucose who had never been pregnant were studied as controls. Using flow cytometric techniques a significant increase in both the number and proportion of activated (Interleukin 2 receptor, CD25) lymphocytes in the peripheral blood of women who had islet cell antibodies and previous impaired glucose tolerance in pregnancy was found (0.14 +/- SE 0.03 x 10(9)/l; 7.1 +/- 1.1%) when compared with normal parous controls (0.09 +/- 0.01 x 10(9)/l; 4.2 +/- 0.6%), p less than 0.01 x 10(9)/l; showed significant increases when compared with nulliparous controls (0.04 +/- 0.01 x 10(9)/l; 2.1 +/- 0.2%), p less than 0.01. No differences were detected between the three groups with respect to total T-lymphocytes (CD3), helper T-lymphocytes (CD4), suppressor cytotoxic T-lymphocytes (CD8), or the inducer of suppressor (Leu 3+/Leu 8+) subset of T-lymphocytes. Three women persistently islet cell antibody positive, two of whom were HLA DR4, showed impaired glucose tolerance at the time of lymphocyte subset analysis, while two further patients, one DR3 and the other DR4, had developed type 1 (insulin-dependent) diabetes. No correlation between increased Interleukin 2 receptor expression and glucose intolerance was demonstrated. We conclude that islet cell antibody positive women with impaired glucose tolerance during pregnancy are at increased risk of later developing type 1 diabetes but that heightened immune activation present in these women is in part a post-pregnancy phenomenon.