OBJECTIVE:To test if a diet of 4.2 MJa24 h as six isocaloric meals would result in a lower subsequent energy intake, or greater energy output than (a) 4.2 MJa24 h as two isocaloric meals or (b) a morning fast followed by free access to food. DESIGN: Subjects were con®ned to the Metabolic Unit from 19:00 h on day 1 to 09:30 h on day 6. Each day they had a ®xed diet providing 4.2 MJ with three pairs of meal patterns which were offered in random sequence. They were: six meals vs two meals without access to additional foods (6vs2), or six meals vs 2 meals with access to additional food (6 vs2 ), or six meals vs four meals (6 vsAMFAST). In the AMFAST condition the ®rst two meals of the day were omitted to reduce daily intake to 2.8 MJ and to create a morning fast, but additional food was accessible thereafter. Patients were con®ned in the chamber calorimeter from 19:00 h on day 2 until 09:00 h on day 4, and then from 19:00 h on day 4 to 09:00 h on day 6. The order in which each meal pattern was offered was balanced over time. MEASUREMENTS: Energy expenditure (chamber calorimetry), spontaneous activity (video) and energy intake (where additional foods were available) during the ®nal 24 h of each dietary component. SUBJECTS: Ten (6vs2), eight (6 vs2 ) and eight (6 vsAMFAST) women were recruited who had a BMI of greater than 25 kgam 2 . RESULTS: From experiment 6vs2 the difference between energy expenditure with six meals (10.00 MJ) and two meals (9.96 MJ) was not signi®cant (P 0.88). Energy expenditure between 23:00 h and 08:00 h (`night') was, however, signi®cantly higher (P 0.02) with two meals (9.12 MJa24 h) compared with six meals (8.34 MJa24 h). The pattern of spontaneous physical activity did not differ signi®cantly between these two meal patterns (P b 0.05). Total energy intake was affected by neither meal frequency in experiment 6 vs2 (10.75 MJ with six, 11.08 MJ with two; P 0.58) nor a morning fast in experiment 6 vsAMFAST (8.55 MJa24 h with six, 7.60 MJ with AMFAST; P 0.40). The total diet of subjects who had a morning fast tended to have a lower percentage of total energy from carbohydrate (40%) than when they had six meals per 24 h (49%) (P 0.05). Subsequent energy balance was affected by neither meal frequency (6vs2; P 0.88, 6 vs2 ; P 0.50) nor a morning fast (P 0.18). CONCLUSIONS: In the short term, meal frequency and a period of fasting have no major impact on energy intake or expenditure but energy expenditure is delayed with a lower meal frequency compared with a higher meal frequency. This might be attributed to the thermogenic effect of food continuing into the night when a later, larger meal is given. A morning fast resulted in a diet which tended to have a lower percentage of energy from carbohydrate than with no fast.