Background The respiratory benefits of muscle strength are well-known in heart-healthy populations, but recommendations and research often focus instead on aerobic fitness (peak oxygen uptake) or total activity. Independent benefits of strength thus may be underestimated, especially in congenital heart disease where perceived dangers of certain types of exercise may outweigh perceived benefits. To assess whether it is plausible that pulmonary benefits of strength in heart-healthy populations also apply in congenital heart disease, we simultaneously correlated these patients’ lung function with fitness, strength, and cardiac diagnosis. Methods Lung function (forced expiratory volume in one second percentage predicted (FEV1%pred)) was modeled as function of handgrip strength, congenital heart disease diagnosis, peak oxygen uptake and the interactions of handgrip with sex and diagnosis in 538 Germans (58% male, ages 6–82 years) in linear models corrected for age, sex, height and weight. Congenital heart disease diagnoses were: complex cyanotic; Fallot/Truncus arteriosus communis (common arterial trunk) (TAC); shunts; transposition of the great arteries (TGA); left heart; and other/none. Results Each kg of handgrip was associated with 0.74% higher FEV1%pred ( p < 0.001) and handgrip explained almost 10% of variance in FEV1%pred. While some groups had higher FEV1%pred than others ( p for global null <0.0001), all experienced similar associations with strength ( p for interaction with handgrip >0.10 for both sex and diagnosis.) Correction for peak oxygen uptake eliminated the association with congenital heart disease, but not handgrip. Conclusion Strength was associated with better lung function in all ages even after correction for peak oxygen uptake, regardless of sex and congenital heart disease. This suggests that strength may be at least as important for lung function as aerobic fitness. Heart-safe strength training may improve pulmonary function in congenital heart disease.