1. The obese gene product leptin, secreted exclusively from adipocytes, was discovered to serve as a satiety factor and to play an important role in regulating body weight. In adults, the serum leptin level reportedly increases with the degree of obesity. Leptin receptors are expressed in various tissues, and recent in vitro studies suggest a role for leptin in haematopoiesis. 2. The present study was designed to clarify the relationship between serum leptin and body mass index, peripheral blood cell counts, serum cholesterol, high-density lipoprotein-cholesterol, insulin and cortisol levels in 299 Japanese male adolescents aged 15-16 years. 3. With simple linear correlation, log [serum leptin] showed a strong correlation with body mass index (r = 0.56), log [insulin] (r = 0.36) and leucocyte count (r = 0.22) (P < 0.001 for all). There were also correlations with systolic blood pressure, erythrocyte count, haematocrit and high-density lipoprotein-cholesterol (P < 0.01 for all). Even after adjustment for body mass index and log [insulin], log [leptin] correlated with leucocyte (P = 0.004) and erythrocyte (P = 0.057) counts. Stepwise multiple regression analyses revealed log [leptin] to correlate significantly with body mass index, log [insulin] and the leucocyte count (P < 0.005 for all, r2 = 0.399). 4. To our knowledge, this is the first clinical study to show the possible association of serum leptin level with blood cell counts, independent of body mass index and serum insulin. We conclude that these data further support a role for leptin in haematopoiesis.