The aim of this study was to assess the effects of a 1-year intensified diet and exercise education regimen on habitual physical activity and aerobic capacity in middle-aged, obese patients with newly-diagnosed Type 2 (non-insulin-dependent) diabetes mellitus. In addition, we analysed whether the level and the changes in physical activity and aerobic capacity are related to the metabolic control of diabetes. After a 3-month basic education programme, 78 patients (45 men, 33 women) were randomly placed in an intervention or conventionally treated group. The intervention group received intensified diet education and continuous encouragement to increase physical activity which was monitored using exercise records and questionnaires. Aerobic capacity was assessed by measuring oxygen uptake at anaerobic threshold and at peak exercise. The proportion of patients with regular recreational exercise increased from 24% to 38% in the intervention men (0.10 less than p less than 0.20), remained at 54% in the conventionally treated men, increased from 53% to 70% in the intervention women (0.10 less than p less than 0.20) and from 31% to 50% (0.10 less than p less than 0.20) in the conventionally treated women. No measurable improvement was found in oxygen uptake in any of the groups. When the groups were combined, HbA1c showed an inverse correlation with oxygen uptake at anaerobic threshold (r = 0.27, p less than 0.01) and maximum oxygen uptake (r = 0.28, p less than 0.01) at 12 months.(ABSTRACT TRUNCATED AT 250 WORDS)