Weight reduction is recommended for the treatment of subjects with insulin resistance (IR) syndrome; however, the relative importance of the decrease in body fat or the negative energy balance achieved during a hypo-energetic diet in the improvement of this metabolic syndrome is still debated. Therefore, we undertook to study their relative impact on amelioration of the metabolic abnormalities associated with IR in obese subjects. Twelve obese subjects (six males and six females, mean.. body mass index 36· 14· 7 kg/m2) aged 38–57 years were investigated. During the first phase they were fed a hypo-energetic diet for 6 weeks (week 0–6). During the second phase, lasting 4 weeks (week 6–10) they consumed an iso-energetic diet. During the third phase (week 10–16) the subjects were put again on a hypo-energetic diet. Insulin sensitivity (SI) was assessed by an insulin-enhanced, frequently sampled iv glucose tolerance test with minimal model analysis. All subjects reduced weight during both hypo-energetic periods: 5· 490· 75 and 2· 320· 37%, means..., P< 0· 005, week 0–6 and 10–16 respectively. One-third of this loss was achieved within the first week of each period. SI increased by 353121 and 14738%(P< 0· 005), means..., at the end of both hypo-energetic periods (week 6 vs 0 and 16 vs 10 respectively). Two-thirds of this improvement were observed within the first week of each period (week 1 vs 0 and 11 vs 10 respectively). During the iso-energetic weight-maintaining period (week 10 vs 6), SI decreased by 43· 57· 9%(P< 0· 002). Serum levels of leptin and triglyceride followed a similar pattern, but to a lesser extent.