Although it has been recognised for a century that an individual's intraocular pressure (i.o.p.) varies over a 24 hr period, the implications for the clinician of this physiological characteristic remain uncertain. While financial constraints nowadays prohibit in-patient assessment of suspect ocular hypertensive cases, the U.K. has witnessed a concurrent increase in the monitoring of at-risk subjects at local centres, including optometric practices. Consequently a description of the pattern of variation of IOP in the at-risk population during normal consulting hours, assessed using a popular model of pneumo-applanation tonometer, could assist the clinician in establishing management and referral protocols. This report describes the results of a single-centre study based on the retrospective analysis of the records of over 1,500 age-matched male and female at-risk but assumed non-glaucomatous subjects. Statistical analysis of the collated data indicated that subject age group was not a major variable: however gender and the time of IOP assessment were both significant features. For recordings between the hours of 09.00 and 18.00 human non-glaucomatous males and females registered an IOP peak in the late morning, succeeded by a lowered value from mid-afternoon. For both male age groups (40-59 and 60+ years) the afternoon mean IOP values were significantly lower than the male morning values and significantly different to the female morning and afternoon means. On the basis of these results it is suggested that male subjects who record borderline or high pressures at an afternoon appointment should be reassessed the following (mid-)morning; also that subsequent reviews of such cases should be scheduled for morning visits. By adopting this approach the likelihood of recording an elevated IOP value is increased and true-positive cases of raised IOP identified. However, during normal consulting hours the present evidence suggests that this temporal precaution need not be observed in female subjects.