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Background Factor V Leiden homozygosity predisposes patients to deep venous thrombosis and major pulmonary thromboembolism. Consequently, factor V Leiden homozygosity could, via unrecognized repeated minor pulmonary thromboemboli, cause chronic pulmonary disease. We tested the hypothesis that factor V Leiden homozygosity is associated with pulmonary symptoms and signs. Methods We studied a general population sample of 9253 individuals from the Copenhagen City Heart Study who were examined in 1991-1994. Of these, 6475 participants were also examined in 1976-1978 and/or 1981-1983. End points were dyspnea and lung function. Results Among 20 factor V Leiden homozygotes, a mean ± SD of 32% ± 11% had severe dyspnea compared with 6% ± 0.3% of 8534 noncarriers (χ2 test; P<.001). The corresponding adjusted odds ratio for severe dyspnea was 5.4 (95% confidence interval, 1.9-15.7). During follow-up, forced expiratory volume in 1 second and forced vital capacity were 5% to 10% lower in homozygotes vs noncarriers (analysis of variance; P = .003 and P = .03). The annual mean ± SD loss of forced expiratory volume in 1 second and forced vital capacity was 39 ± 8 mL/y and 35 ± 8 mL/y in homozygotes vs 21 ± 10 mL/y and 15 ± 10 mL/y in noncarriers (t test; P = .03 and P = .04), respectively. Factor V Leiden heterozygosity (n = 699) did not influence pulmonary symptoms and signs. Conclusion We demonstrate a previously unrecognized clinical presentation of factor V Leiden homozygosity with severe dyspnea and decreased pulmonary function. [ABSTRACT FROM AUTHOR]