Blood pressure (BP) levels, particularly in hypertensives, often fall on admission to hospital. Whether such a BP fall also occurs for BP measured automatically without the presence of medical and nursing staff and hence without the alerting reaction has not previously been studied. Twenty-two subjects, mean age 80 ± 7 years (BP range 110–190/60–110 mmHg) underwent nurse-taken BP measurements and 24-hour non-invasive BP monitoring on admission to hospital and again after 5 days. Nurse-taken systolic BP (SBP) levels on admission fell significantly from 149 ± 21 mmHg to 137 ± 16mmHg (p<0.01) on day 2, to 131 ± 16mmHg on day 5 (p <0.05). Mean diastolic BP (DBP) levels showed a less marked fall over 5 days from 87 ± 12 mmHg to 84 ± 11 mmHg (p < 0.05). Mean 24-hour BP levels showed no significant change from admission to day 5 (131 ± 17/72 ± 12 mmHg vs. 131 ± 17/72 ± 10 mmHg, respectively). In addition, nurse-taken SBP on day 5 was similar to mean 24-hour and daytime SBP taken on day 1 or day 5. Using shorter periods of BP monitoring gave similar mean BP levels to the full mean 24-hour BP level although BP reproducibility was poorer, even with more than 10 hours of BP monitoring. A period of automatic BP monitoring on admission to hospital will rapidly reveal a patient's sustained hospital BP level.