['To examine the association between cardiovascular fitness and homocysteine levels in adolescents.', 'Cross-sectional study.', 'Madrid, Murcia, Granada, Santander, and Zaragoza, Spain.', 'One hundred fifty-six Spanish adolescents (76 boys and 80 girls) aged (mean +/- SD) 14.8 +/- 1.4 years.', 'Cardiovascular fitness was measured by the 20-m shuttle run test. Pubertal stage, birth weight, smoking status, and socioeconomic status were determined, and the sum of 6 skinfold thickness measurements, and serum folic acid and vitamin B(12) levels were measured. Methylenetetrahydrofolate reductase (MTHFR; 677C>T genotype) polymorphism was done by DNA sequencing.', 'Fasting homocysteine levels.', 'Mean values of homocysteine were significantly higher in the MTHFR 677CT and TT genotype subgroups compared with the CC genotype subgroup in adolescent boys, whereas in adolescent girls, mean values of homocysteine were significantly higher in the MTHFR 677CT and TT genotype subgroup compared with the CC and CT genotype subgroups. Multiple regression analyses showed that cardiovascular fitness was significantly associated with homocysteine levels in female adolescents after controlling for potential confounders including the MTHFR 677C>T genotype (beta = -0.40; semipartial correlation = -0.35; P = .007). No associations were found between cardiovascular fitness and homocysteine levels in male adolescents (beta = 0.12; semipartial correlation = 0.08; P = .51).'] Is cardiovascular fitness negatively associated with homocysteine levels in female adolescents?