OBJECTIVE: To non-invasively study the effects of continuous positive airway pressure breathing (CPAP) on renal vascular resistance in normal subjects and renal allograft recipients, in other words those with with denervated kidneys. We could then ascertain the influence of renal innervation on any resulting changes in renal haemodynamics. METHODS: Ten healthy volunteers and six renal transplant patients were studied. Using Doppler ultrasonography, the pulsatility index (PI), an index of renovascular resistance, was measured at incremental levels of CPAP (0, 2.5, 5.0 and 7.5 cmH2O). RESULTS: In both groups, the PI increased significantly between 0 and5.0 cmH2O CPAP, with a further increase at 7.5 cmH2O CPAP. CONCLUSIONS: We found that CPAP at 5.0 and 7.5 cmH2O caused a significant increase in renovascular resistance in both normal and renal transplant patients. There was no difference in the degree of rise in renovascular resistance between both groups, indicating that the renal nerves do not play a role in altering renal vascular resistance with the application of CPAP.