OBJECTIVE: To study the effect on the early (cephalic) phase of diet-induced thermogenesis (EDT) of palatable vs unpalatable food, in nonobese and obese man. SUBJECTS: Twenty-four nonobese volunteers and 19 obese clinic patients. DESIGN AND MEASUREMENTS: A palatable, liquid formula meal of Ensure (1048 KJ, 450 ml), and of Ensure made unpalatable by addition of aqueous KCl, were sipped on nonconsecutive mornings. O 2 consumption (ml/min) was measured before, and starting 30, 60 and 90 min after beginning the test meal, from which EDT was calculated as KJ/min. RESULTS: Palatability of the test meal signi®cantly increased EDT (palatability effect, P 0.004) but obesity status per se, did not affect EDT. Nevertheless, the effect of palatability on EDT was dependent on obesity status, being seen only in the nonobese. EDT was signi®cantly greater in the nonobese after the palatable than the unpalatable meal: (mean 6 s.e.m.) 2.45 6 0.14 vs 1.83 6 0.14; P`0.0001, but not in the obese: 1.93 6 0.28 vs 1.73 6 0.20; P`0.21. Therefore only after the palatable meal was EDT less in the obese compared with the nonobese: P`0.05.The threshold for the unpleasant taste of added KCl was 31% higher in the obese than the nonobese: 4.2 6 0.4 vs 3.2 6 0.2 [g KCl]; P`0.025. CONCLUSIONS: The early (cephalic) phase of dietary thermogenesis (EDT) is signi®cantly increased in the nonobese by palatability, but not in the obese, so that only after a palatable meal is EDT less, or`de®cient,' in the obese compared with the nonobese. Also, the obese have a higher threshold for the unpleasant taste of KCl (in Ensure) than the nonobese.