The purpose of this study was to assess the effectiveness of an 8 week aerobic exercise class for women on changes in their physical activity behaviour. This was a descriptive, retrospective and comparison study. Because the data were collected after an 8 week aerobic exercise class, there were two groups of subjects. Subjects in the exercise group had attended exercise classes, but subjects in the control group had not. Results showed statistically significant differences between the two groups in the frequency/week of moderately intense exercise-related activities, caloric expenditure/week in all exercise-related activities and caloric expenditure/week in moderately intense exercise-related activities. However, there was no statistically significant difference in the frequency/week of all exercise-related activities. The result that women in the exercise group practised physical activities more than women in the control group might be related to their high exercise self-efficacy expectations.