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To examine the relationship between estimated maximal cardiorespiratory fitness (CRF) and metabolic syndrome (MetSyn).We performed a cross-sectional analysis of 38,659 Aerobics Center Longitudinal Study participants seen between January 1, 1979, and December 31, 2006, to examine CRF levels defined as low (lower 20%), moderate (middle 40%), and high (upper 40%) of age- and sex-specific distributions vs National Cholesterol Education Program–derived MetSyn expressed as a summed z-score continuous variable. We used a general linear model for continuous variables, the χ2 test for distribution of categorical variables, and multiple linear regression for single and cumulative MetSyn scores adjusted for body mass index, smoking status, alcohol intake, and family history of cardiovascular disease.We observed significant inverse trends for MetSyn vs CRF in both sexes (P for trend <.001). The CRF associations vs individual components were as follows: waist circumference–men: β=−.14, r2=0.78; women: β=−.04, r2=0.71; triglycerides–men: β=−.29, r2=0.18; women: β=−.17, r2=0.18; high-density lipoprotein cholesterol–men: β=.25, r2=0.17; women: β=.08, r2=0.19; fasting glucose–men: β=−.09, r2=0.09; women: β=.09, r2=0.01; systolic blood pressure–men: β=−.09, r2=0.09; women: β=−.01, r2=0.21; and diastolic blood pressure–men: β=−.07, r2=0.12; women: β=−.05, r2=0.14. All associations except for systolic blood pressure (both sexes) and glucose (women) are significant (P<.001).Cardiorespiratory fitness demonstrated a strong inverse relationship with MetSyn in both sexes, with the strongest single associative component being waist circumference.