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To see whether measurements of ambulatory blood pressure during short-term daytime and night-time periods can represent complete daytime and night-time pressures accurately.Short-term measurements would be less uncomfortable for patients, easier to perform and could lead to fewer missing values, outliers or artefacts than full-day measurements, especially when repeated monitorings are required.Ambulatory blood pressure was measured every 15 min for 24 h in 254 subjects with normal or borderline office blood pressure. Each pressure profile included at least 80 valid readings. Mean blood pressures for different 1-, 2-, 3-, 4-, 5- and 6-h spans were calculated and compared with mean daytime and night-time values using paired Student's t-test.One or two-hour spans of daytime blood pressure poorly represented mean daytime pressure. In contrast, 4-h readings, selected between 1000—2200 h represented daytime blood pressure with good accuracy. Over the total sample, 4-h mean blood pressure readings from 1000—2200 h differed from daytime readings by <2mmHg and 2-h mean readings from 0300—0700 h differed from mean night-time readings by <1 mmHg.We suggest that 4-h measurements of ambulatory blood pressure during the daytime and 2-h measurements at night (with time spans selected as indicated as above) may be of value.