Although hypophosphatemia has been implicated as a cause of respiratory failure, its impact on respiratory muscle function in patients hospitalized for other reasons remains to be determined. Maximal inspiratory pressures (MIP) and maximal expiratory pressures (MEP) were measured at the bedside in 23 hospitalized patients with serum phosphate levels less than 2.5 mg/dl, and these measurements were repeated daily during phosphate repletion until serum phosphate levels reached the normal range. A control group consisted of 11 normophosphatemic inpatients. Sixteen of 23 hypophosphatemic patients, but none of the control patients (p less than 0.001), exhibited respiratory muscle weakness, defined as a MIP less than 40 cm H2O or a MEP less than 70 cm H2O. The mean initial MIP and MEP values were also significantly lower for the hypophosphatemic group. A significant correlation existed between initial phosphate level and initial MIP value (r = 0.50, p less than 0.02). With phosphate repletion, mean +/- SD MIP increased from -37 +/- 26 cm H2O to -49 +/- 24 cm H2O (p less than 0.003) and MEP from 60 +/- 20 cm H2O to 69 +/- 19 cm H2O (p less than 0.02). It is concluded that respiratory muscle weakness is common among hypophosphatemic patients and improves with phosphate repletion.