Respiratory muscle strength during acute upper respiratory tract infection (URI) was assessed in patients with various forms of neuromuscular disease. Vital capacity (VC), oxygen saturation, end-tidal PCO2, maximal inspiratory pressure (MIP), and maximal expiratory pressure (MEP) were determined in 25 stable patients with various forms of neuromuscular disease. Thirteen episodes of URI developed in 10 patients. Respiratory parameters were reassessed within 24-36 h following the onset of symptoms in each patient. In patients with URI, mean baseline VC, MIP, and MEP were 1.16 L +/- 0.14, 49.2 cm H2O +/- 6.8, and 35.5 cm H2O +/- 3.8 and fell to 1.01 L +/- 0.15, 37.1 cm H2O +/- 6.2, and 25.5 cm H2O +/- 3.0 during URI (p < 0.05 for each), respectively. Mean baseline PCO2 and oxygen saturation were 39.1 mm Hg +/- 1.1 and 95.1% +/- 1.0, and during URI, 43.9 mm Hg +/- 2.1 (p < 0.05) and 95.0% +/- 1.0 (NS), respectively. Five episodes of significant hypercapnia were observed in 4 patients. All parameters returned to near baseline values following recovery. We conclude that patients with various forms of neuromuscular disease develop reductions in respiratory muscle strength in association with URI. Unlike normal subjects, however, these decrements in respiratory muscle function may result in symptoms of shortness of breath, reductions in vital capacity, and acute hypercapnia in this patient population.