This study investigated whether a drug therapy-induced reduction in nocturnal blood pressure (BP) was associated with decreased sleep apnea activity. Two polysomnographies from 54 hospitalized male hypertensive, obstructive sleep apnea patients were analyzed in a double-blind, randomized, parallel-group trial of the angiotensin-converting enzyme inhibitor cilazapril (C), 2.5 mg once daily, or placebo (P). Blood pressure was measured by means of an intra-arterial catheter. Compared with P, C lowered mean arterial BP during non-rapid eye movement (NREM) (-8.3 +/- 10.7 mm Hg, P = .05) and REM sleep (-8.6 +/- 10.1 mm Hg, P = .02). Respiratory disturbance index (-8.6 +/- 3.2 events/h of sleep (n/h), P = .01) and apnea index (AI) (-6.6 +/- 3.0 n/h, P = .04) during NREM sleep were lowered by C and, to a lesser extent, by P (-5.9 +/- 3.2 n/h, P = .07 and -5.0 +/- 3.6 n/h, P = .18, respectively). The effect on AI and hypopnea index (HI) during REM sleep was not significant for C (-5.9 +/- 3.4 and 0.1 +/- 2.0, NS, respectively) and P (-2.6 +/- 3.9 and 1.6 +/- 2.0, NS, respectively). There was a significant linear correlation between the change in REM systolic BP and the change in REM AI (r = 0.28, P = .04); the mean BP change correlated negatively with the change in HI (-0.28, P = .04). During NREM sleep there was no significant correlation between changes in BP and the treatment effects on sleep apnea activity. Blood pressure reduction after short-term antihypertensive treatment did not affect sleep disordered breathing during NREM sleep. Reduced BP was associated with a weak reduction of AI and a slight increase of HI during REM sleep. It appears that elevated BP contributes only marginally to sleep apnea severity in hypertensive patients with obstructive sleep apnea.