The low affinity receptor for IgE, CD23, is expressed on lymphocytes among other cell types. The purpose of the present study was to assess serum sCD23 levels and CD23 expression on peripheral blood mononuclear cells (PBMC) in people at increased risk of developing Type 1 diabetes mellitus and in diabetic patients. Serum sCD23 levels were significantly higher in first-degree relatives of Type 1 patients (median: 3.2 U ml(-1)) (p < 0.001) and in newly diagnosed (median: 3.3 U ml(-1)) (p < 0.001) and long-standing (median: 2.5 U ml(-1)) (p = 0.01) Type 1 diabetic patients than in controls (median: 1.2 U ml(-1)). Newly diagnosed patients showed higher levels than those with long-standing disease (p = 0.026). Moreover the percentage of B cells expressing CD23 were significantly higher in first-degree relatives (median: 48.6%) (p < 0.001) and in newly diagnosed (median: 58%) (p < 0.001) and long-standing (median: 44.8%) (p = 0.03) Type 1 diabetic patients than in controls (median: 28.5%). The increased sCD23 levels and the increased number of cells expressing CD23 observed in subjects at increased risk of Type 1 diabetes and diabetic patients may be indicators of Th2 activity in Type 1 diabetes.