The purpose was to examine (1) the effect of cycling to work on physical performance; (2) the minimum weekly energy expenditure needed for fitness improvement based on the dose-response relationship between total caloric expenditure and fitness changes. Healthy, untrained men and women, who did not cycle to work, participated in a 1-year intervention study. Sixty-five subjects were asked to cycle to work at least three times a week. Fifteen subjects were asked not to change their living habits. All measurements were performed on three consecutive occasions, with 6 months in between. Maximal external power (P(max)), heart rate, respiratory exchange ratio and peak oxygen uptake (VO(2peak)) were assessed. Cycling characteristics and leisure time physical activities were reported in a dairy. A significant change over time between both groups was seen for VO(2peak) (/kg) in the total group and the women and for P(max) in the total group. Correlations were found between VO(2peak) (/kg) (r>/=0.40) and kcal/week and min/week. Preliminary results indicate that the minimum expended energy needed for the improvement of indexes of fitness is higher for men compared with women.