Physical inactivity contributes to premature mortality and morbidity and increasing prevalences of overweight and obesity in industrialized countries. Computer use is an increasingly common sedentary behaviour, potentially displacing physical activity. Physical activity and computer use were examined in 697 young adults (18-30 years). Energy expenditure estimates were derived from self-reported walking, moderate, and vigorous activity; participants were classified as sedentary, low, moderate, or high in their level of activity. For multivariate analyses, two categories of physical activity were used: inactive (sedentary/low activity; < 800 kcal.week-1) or active (moderate/high activity; > or = 800 kcal.week-1). Time spent in computer-related activities was summed, and computer use tertiles calculated (< 3 hours.week-1; 3-8 hours.week-1; > 8 hours.week-1). Those in the highest tertile of computer use were most likely to be inactive (p = 0.003) and most likely to report computer use as a barrier to physical activity (p < 0.001). The majority of those in the top two tertiles of computer use, and of the inactive, preferred obtaining information from computers than from conventional print media. These findings suggest that computer use plays a significant role in the discretionary time of young adults and is negatively associated with physical activity. Computer-mediated communication has potential in disseminating interventions to increase physical activity in young adults.