The impact of continuous positive airways pressure (CPAP) on diuresis and urine concentration was studied in newborn preterm infants with respiratory insufficiency with and without pulmonary X-ray changes. Urine output and osmolality, sodium balance and blood pressure were measured before, during and after CPAP application. In all infants urine osmolality rose and urine output decreased significantly when high CPAP (8 cm H2O) was applied, especially in infants without radiographic pulmonary changes. Three infants also developed hyponatremia at high CPAP levels. The results indicate that high CPAP levels per se may affect fluid balance inadvertently. Therefore fluid intake and urine output should be monitored closely during CPAP treatment.