Heritability of forced expiratory volume in one second (FEV1), forced vital capacity (FVC), and peak expiratory flow (PEF) has not been previously addressed in large twin studies. We evaluated the genetic contribution to individual differences observed in FEV1, FVC, and PEF using data from the largest population-based twin study on spirometry. Specially trained lay interviewers with previous experience in spirometric measurements tested 4,314 Danish twins (individuals), 46–68 years of age, in their homes using a hand-held spirometer, and their flow-volume curves were evaluated. Modern variance component sex-limitation models were applied to evaluate possible genetic differences between the sexes for FEV1, FVC, and PEF. Estimates were adjusted for age, height, and smoking. For FEV1, additive genetic effects of 61% (95% CI 56–65) were observed. For FVC, the additive genetic contribution was 26% (3–49%) and the dominant genetic contribution was 29% (4–54%). For PEF, our models showed an additive genetic contribution of 43% (31–52%) for men, but genetic influences were not significant in women. We found no significant differences between dizygotic same-sex twins and dizygotic opposite-sex twins for FEV1, FVC, and PEF, suggesting absence of qualitative genetic differences between the sexes. Sex-difference heritability for PEF suggested possible quantitative genetic differences between the sexes for this index. Genetic effects contributed significantly to individual differences observed in FEV1, FVC, and PEF. Qualitative sex differences were absent for all spirometric measures, while quantitative sex differences were observed only for PEF, with heritability being substantial in men but negligible in women.