Results of studies on diet-induced thermogenesis in obese persons are contradictory. A number of factors have been postulated to mediate the obligatory and facultative component of thermogenesis. This study was designed to investigate some further factors mediating the carbohydrate-induced thermogenesis in obese women. In 13 obese women, thermogenic responses to glucose and fructose were compared and related to subsequent hormonal changes. The thermogenic effect after fructose ingestion was significantly (p<0.006) higher in comparison with glucose, despite lower values for both glucose and insulin concentrations. Carbohydrate oxidation was significantly higher after fructose (81+/-7 E% vs 62+/-10 E% p<0.01) while oxidation of fat was lower (10+/-9 E% vs 21+/-12 E% p<0.01). These effects may partly be due to the de novo lipogenesis and/or to changes in cellular metabolism. No clear relationship could be found between thermogenesis and the activity of the sympathetic nervous system, as expressed by urinary catecholamine levels. These results indicate that not insulin but the cellular rate of carbohydrate metabolism is responsible for the thermogenic response to different carbo-nutrients.