AIMS: The role of sex steroids in glucose and insulin metabolism in men remains unclear. To investigate the effects of sex steroids and oestrogen on insulin sensitivity in men, we studied two male adults with aromatase deficiency (subject 1 and subject 2). METHODS: The effects of transdermal oestradiol (tE(2)) treatment at different dosages on insulin sensitivity were studied before tE(2) treatment (phase 1), and after 6 months (phase 2) and 12 months of tE(2) treatment (phase 3) by means of homeostasis model assessment-insulin resistance (HOMA-IR) and Quantitative Insulin Sensitivity Check Index (QUICKI), insulin tolerance test (ITT), and oral glucose tolerance test (OGTT). The latter was performed only in subject 1, as subject 2 suffered from Type 2 diabetes. RESULTS: The restoration of normal serum oestradiol led to improved insulin sensitivity, as shown by changes in HOMA-IR and QUICKI. The ITT provided evidence of improved insulin sensitivity during tE(2) treatment. Insulin secretion after OGTT was reduced during tE(2) treatment in subject 1. After 12 months of tE(2) treatment, insulin sensitivity was improved compared with in phases 1 and 2. CONCLUSIONS: The study suggests a direct involvement of oestrogens in insulin sensitivity, and supports a possible role of oestradiol : testosterone ratio, which may be as influencial as the separate actions of each sex steroid on glucose homeostasis.