Objective: Despite the health benefits, many people do not undertake regular exercise. This study investigated the effects of moderate-intensity exercise on cardiorespiratory fitness (lung age, blood pressure and maximal aerobic power, VO[subscript 2]max), serum lipids concentration and body mass index (BMI) in sedentary overweight/obese adults consuming a calorie-restricted low-fat diet. Design: Randomized diet and exercise intervention. Setting: Lincolnshire, UK. Methods: Sixty overweight/obese (BMI greater than or equal to 25kgm[superscript -2]) adults were randomized to either a calorie restricted low-fat diet (20 per cent of total energy as fat) or the same diet with the addition of moderate-intensity physical exercise (30 minutes, twice a week) for eight weeks; 20 completed the study. Participants' serum lipids concentrations, BMI, blood pressure, resting pulse rate, VO[subscript 2]max and lung age were measured before the start of the intervention and during the fourth and eighth weeks. Results: Reductions in blood pressure (10 per cent versus 1 per cent), pulse rate (13 per cent versus 8 per cent) and weight (5 per cent versus 2 per cent) were greatest for the diet with exercise group. Exercise resulted in a significant (p less than or equal to 0.05) increase in average VO[subscript 2]max (by 17 per cent) and reduction in average lung age by about 19 years. Further, reduction in participants' lung age ranged from 1 to 37 years. However, there was no significant difference in BMI, blood pressure and serum lipids concentration between groups. Conclusion: Although exercise on most days of the week would result in maximum health benefits, 30 minutes of moderate-intensity exercise twice a week could significantly improve cardiorespiratory fitness (blood pressure and lung age) and the risk of cardiovascular diseases in previously sedentary overweight/obese adults. (Contains 2 figures and 2 tables.)