We assessed the effects of ingesting caffeine before passive heat loading (PHL) on serum leptin and sweating response, which are both physiological responses associated with energy expenditure. The subjects were nine male university students (age, 24.1 +/- 3.5 years; height, 173.4 +/- 7.6 cm; weight, 69.2 +/- 5.7 kg; maximal oxygen consumption, 48.6 +/- 4.7 ml a <...aEuro parts per thousand kg(-1) a <...aEuro parts per thousand min(-1)). This study used a within-subject, random, crossover design. Tests were performed twice at the same time (2-5 p.m.) at a 1-week interval following 3 mga <...kg(-1) caffeine ingestion (Caff-I) or not (No-Caff). PHL included a half bath in hot water (42 +/- 0.5 A degrees C for 30 min) in a thermoneutral climate chamber (25 A +/- 0.5 A degrees C, 60 A +/- 3 % relative humidity, < 1 m/s air velocity). After PHL, blood levels of leptin and free fatty acids were significantly higher in the Caff-I compared to those in the No-Caff after PHL (P < 0.01). Waist circumference and whole-body sweat loss volume were significantly higher in the Caff-I compared to those in the No-Caff (P < 0.001). Mean active sweat gland density was significantly higher in the Caff-I compared to those in the No-Caff at 10 min during PHL (P < 0.001). The results suggest that ingesting caffeine before PHL is more energy efficient than that of a single PHL.