Pneumocystis colonization has been associated with severity of chronic obstructive pulmonary disease (COPD). The relationship of Pneumocystis antibody status to COPD severity has not been investigated, but antibody levels might relate to both colonization susceptibility and COPD progression. We investigated anti-Pneumocystis antibody titers and airway obstruction in a cohort of patients with COPD. Undetectable anti-Pneumocystis antibody titer was an independent predictor of more-severe airway obstruction, although use of inhaled corticosteroids is a possible confounder of this effect.