To assess the reliability of the Del Mar Avionics indirect blood pressure (BP) monitoring system 15 hypertensive patients were concomitantly studied with the intraarterial Oxford system. During daytime a statistically significant correlation was found between the BP values obtained with the two systems. On the contrary during the night this correlation in most cases was not significant. However the histograms of the between-method differences were similar during the day and the night, showing that the Del Mar Avionics Pressurometer (P) performs equally well throughout the 24 hours. It is concluded that the P is reliable for continuous indirect BP monitoring and suitable for the evaluation of the efficacy of antihypertensive treatments.