The effects of dieting and exercise on RMR, body composition and maximal aerobic power were studied in 12 obese women. The subjects were paired on the basis of their body mass index and divided into a diet (D) and a diet + exercise group (DE). The treatment consisted of a 5-week period with a low-energy formula diet of 2.9 MJ and an 8-week period with a mixed diet of 1.7 MJ supplemented with 1.8 MJ normal foodstuffs. DE trained 4 h per week at 50-60 per cent of their maximal aerobic power with aerobics and fitness exercises. Body composition was determined by hydrostatic weighing and RMR was measured from 03.00 to 06.00 hours in a respiration chamber. Maximal aerobic power was measured on a continuously braked ergometer. The measurements were done at week 0, after 4 weeks (week 5), and after 12 weeks (week 13). Weight loss after 4 weeks was 8.2 kg (DE) and 7.9 kg (D) and after 12 weeks 13.2 kg (DE) and 12.2 kg (D). There were no significant differences between the groups. Fat loss was also not statistically different between DE (6.7 and 10.9 kg) and D (6.0 and 9.4 kg). Both groups showed a significant decrease in RMR per kilogram FFM after 12 weeks (DE: 18.2 per cent and D: 26.5 per cent). There was also a significant decrease in RMR for D (19.9 per cent) after 4 weeks but not for DE (12.2 per cent).(ABSTRACT TRUNCATED AT 250 WORDS)