BACKGROUND: Habitual physical activity (HPA) is believed to contribute to overall fitness in CF, however little is known about HPA patterns in adults. METHODS: Adults with CF were recruited from a tertiary hospital outpatient clinic and were compared with controls without CF. HPA was measured as MET·minutes·week(-1) using the long-form International Physical Activity Questionnaire. The relationship between HPA and lung function was explored. RESULTS: CF-group, n=101 [45% females, mean(sd) age=29(9), FEV1 % pred=60(23)] and controls, n=35 [69% females, age 32(10), FEV1 % pred=101(130)]. Both groups reported similar levels of moderate and vigorous activities but the CF-group accumulated significantly less total HPA than controls, mean(sd)=5309(6277) vs. 7808(5493), due to less HPA associated with work, 1887(4285) vs. 3707(5292) and transport 613(1018) vs. 1315(1123). Females with CF showed low to moderate correlations of HPA with lung function (R from 0.30 to 0.42, p<0.05). CONCLUSIONS: Adults with CF accumulate less HPA than non-CF peers. Work and transportation form important targets through which physical activity may be accumulated to supplement prescribed exercise. In females with CF, declining physical activity seen in older adolescents carries into adulthood, which may have implications for wellbeing and outcome.