The effect of meal frequency on the thermic effect of food (TEF) was studied in six obese boys and five obese girls (age: mean +/- SE, 12.7 +/- 0.6 yr). Post-absorptive and postprandial resting energy expenditure (REE) were monitored continuously by indirect calorimetry. The children consumed one large liquid meal (LM) or three consecutive small meals (SM) at 1.5 h intervals on subsequent days. The first mode of nutrient intake was determined random. The energy content of the LM and one SM was tailored to provide 30% and 10% of the 24 h postabsorptive REE, respectively. The postprandial changes in REE were monitored for 6 h. The postabsorptive REE (mean +/- SE) was 4.86 +/- 0.28 and 4.9 +/- 0.27 kJ/min before the LM and SM, respectively. REE, respiratory quotient, plasma glucose and insulin concentrations increased sooner, steeper and higher with the LM than with the SM. The magnitude of the TEF was greater (p < 0.02) after the LM (11.9 +/- 1.3%) than after the SM (8.5 +/- 0.7%). It is concluded that the frequency of food consumption influences the immediate thermogenic response as well as the changes in respiratory quotient, glycaemia and insulinaemia. However, the complex effect of different meal frequencies on the overall energy balance of obese patients cannot be answered on the basis of the present results.