Basal metabolic rate (BMR) was measured on 2 consecutive days with an open-circuit, indirect calorimeter in 16 healthy subjects with trisomy 21 Down's syndrome (DS) aged 22-35 years and 11 healthy control volunteers aged 20-31 years. BMR measurements did not differ on consecutive days in either the DS or control groups. Absolute BMR, BMR corrected for surface area, and BMR corrected for lean body mass did not differ between the DS subjects and the volunteers. The per cent of predicted BMR also did not differ between the two groups. These results suggest that systems controlling basal metabolism are not affected by extra genomic material from chromosome 21.