This study compared the effects of short and long bouts of brisk walking in sedentary women. Forty seven women aged 44.4 ± 6.2 yr (mean ± SD) were randomly assigned to either three 10-min walks per day (short bouts), one 30-min walk per day (long bouts) or no training (control). Brisk walking was done on 5 d·wk-1, at 70 to 80% of maximal heart rate, typically at speeds between 1.6 and 1.8 m·s-1 (3.5 and 4.0 mph), for 10 wk. Subjects agreed not to make changes to their diet. Twelve short-bout walkers, 12 long-bout walkers, and 10 controls completed the study. Relative to controls, ˙VO2max (short-bout, +2.3 ± 0.1 mL·kg-1·min-1; long-bout, +2.4 ± 0.1 mL·kg-1·min-1; controls, -0.5 ± 0.1 mL·kg-1·min-1) and the ˙VO2 at a blood lactate concentration of 2 mmol·L-1 increased in walkers (bothP < 0.05), with no difference in response between walking groups. Neither heart rate during standard, submaximal exercise nor resting systolic blood pressure changed in a different way in walkers and controls. The sum of four skinfold thicknesses decreased in both walking groups (P < 0.05) but body mass (short-bout, -1.7 ± 1.7 kg; long-bout, -0.9± 2.0 kg; controls, +0.6 ± 0.7 kg) and waist circumference decreased significantly only in short-bout walkers. Changes in anthropometric variables did not differ between short- and long-bout walkers. Thus short bouts of brisk walking resulted in similar improvements in fitness and were at least as effective in decreasing body fatness as long bouts of the same total duration.