We report here the design and baseline data from Project Active, a 2-yr randomized trial designed to compare the effectiveness of a Lifestyle physical activity intervention with the traditional Structured exercise prescription approach. Primary outcome measures are energy expenditure in physical activity (estimated by kcal per kilogram of body weight of energy expenditure) and cardiorespiratory fitness (measured by maximal oxygen uptake). The participants, 235 initially sedentary and apparently healthy adults, were randomized into either the Lifestyle or Structured intervention groups. The Lifestyle treatment consists of a personalized approach that accounts for an individual's motivational readiness and preferences for integrating physical activity into daily routines. The Structured approach is the familiar exercise prescription that is based on a frequency, intensity, and duration formula. The primary hypothesis to be tested is that there will be a difference in physical activity and cardiorespiratory fitness measures between the two conditions at the end of 24 months. The secondary hypothesis is that both groups will make significant improvements from baseline in physical activity and cardiorespiratory fitness at the end of 6 months. Six months of active intervention are followed by 18 months of a tapered follow-up maintenance intervention in both groups. Primary outcome measures are measured after 6 and 24 months.