Rationale: Although respiratory dysfunction is common in chronic spinal cord injury, determinants of longitudinal change in FEV₁ and FVC have not been assessed. Objectives: Determine factors that influence longitudinal lung function decline in spinal cord injury. Methods: 174 male participants (mean age of 49 and 17 years after injury) completed a respiratory questionnaire and underwent spirometry over an average follow-up of 7.5 years (range: 4-14). Measurements and Main Results: In multivariate models longitudinal decline in FEV₁ was significantly related to continued smoking, persistent wheeze, an increase in body mass index, and respiratory muscle strength. Aging was associated with an accelerated decline in FEV₁ (for ages <40, 40-60, >60 years, -27, -37 and -71 ml/yr, respectively). Similar effects were observed for FVC. Conclusions: Longitudinal change in FEV₁ and FVC was not directly related to level and severity of spinal cord injury, but was attributable to potentially modifiable factors in addition to age. These results suggest that weight control, smoking cessation, trials directed at the recognition and treatment of wheeze and efforts to improve respiratory muscle strength may slow lung function decline following spinal cord injury.