The authors examined the prospective associations between marital status transitions and changes in fitness in men and women. Between 1987 and 2005, a total of 8,871 adults (6,900 men) aged 45.6 (standard deviation, 9.1) years were examined at the Cooper Clinic, Dallas, Texas; the median follow-up was ∼3 years. Marital transition categories (from single to married, married to divorced, divorced to remarried) were derived from self-reported marital status at baseline and follow-up. Fitness (maximal oxygen consumption) was assessed by a maximal treadmill test. Analyses were adjusted for baseline levels and changes in body mass index, physical activity, smoking, alcohol consumption, and major chronic diseases. Compared with the corresponding "control" groups (remaining single, married, or divorced), transitioning from being single to married was associated with a reduction in fitness in women (P = 0.03); divorce was associated with an increase in fitness in men (P = 0.04); and remarriage was associated with a reduction in fitness in men (P = 0.05). The authors conclude that the transitions to being married (from single to married or from divorced to remarried) are associated with a modest reduction, while divorce is associated with a modest increase in fitness levels in men. Study results suggest that these patterns may be different in women, but further research is required to confirm this.