We examined relationships between self-reported physical activity and physical fitness in a group of 202 male military officers, 36 to 51 years of age. Physical activity was estimated from the Minnesota Leisure Time Physical Activity Questionnaire and classified as heavy, moderate, or light depending on the intensity of individual activities. Directly measured components of physical fitness included aerobic capacity (treadmill walking peak VO2), body composition (densitometry), muscle strength (isokinetic mode), and flexibility (bend-and-reach test). Both total and heavy activity were significantly (P < .05) related to aerobic capacity, body fat, fat-free mass, upper body strength, and flexibility. Moderate and light activity were not related to the fitness measures. We divided subjects into quartiles of physical activity and corrected the fitness measures for age and cigarette smoking status. Subjects with more total and heavy activity had higher aerobic capacity and lower body fat; subjects with more heavy activity also had greater upper body strength. Again, moderate and low activity were not related to any fitness measure. These data indicate that heavy physical activity is related to various components of physical fitness even after correction for age and smoking status in a group of men who are healthy, generally very active, and fit.