This investigation identified demographic and health-related characteristics of 1,877 sedentary, underactive, and regularly active individuals aged 50 to 65 randomly sampled from a northern California city. Physiological and psychosocial information was available in greater detail for a subsample (n = 327) of sedentary and underactive persons who were subsequently enrolled in a randomized, controlled, clinical trial (SSHIP). Results suggested that unmarried men, women reporting poor health, and smokers were most likely to be completely sedentary. Sedentary and underactive individuals responded differently to two recruitment strategies designed to attract participants into SSHIP. In addition, the initially sedentary participants had significantly lower adherence rates across the 1-year exercise trial compared to the initially underactive regardless of either the format or intensity of the program. These data underscore the utility of differentiating between levels of less-than-optimal physical activity in formulating campaigns promoting physical activity as well as designing exercise interventions.