A persistently low population level of physical activity is a challenge for public health. Data on cost effectiveness of environmental interventions are needed to inform the development and implementing of such interventions. To conduct cost-effectiveness analysis of bicycle/pedestrian trails. The costs of trail development and number of users of four trails in Lincoln, NE, were obtained. The costs were adjusted to 2003 dollars. The physical activity-related outcomes/items are number of users who were more physically active since they began using the trails, number of users who were physically active for general health, and number of users who were physically active for weight loss. Cost-effectiveness measures were derived. Sensitivity analysis was performed. The annual trail development cost US$289,035, 73% of which was construction cost. Of the 3,986 trail users, 88% were active at least 3 days a week. The average annual cost for persons becoming more physically active was US$98 (range US$65-253); the cost was US$142 (range US$95-366) for persons who are active for general health, and US$884 (range US$590-2,287) for persons who are active for weight loss. This analysis provides basic cost-effectiveness measures of bicycle/pedestrian trails. Policymakers can use this information in making resource allocation decisions.