BACKGROUND: Pulsatile ocular blood flow (POBF) measurement is a new parameter to aid the understanding of the aetiology of low-tension glaucoma. There has been one study reporting the diurnal variation of POBF. However, that study involved eight subjects only. This study investigated the daytime variation of POBF with a greater sample size. METHODS: Twenty-four young Chinese subjects (12 males and 12 females) were recruited. The mean age of our subjects was 23.6 years and only the right eye was analysed. All the subjects were screened for glaucoma and the POBF was measured at three-hourly intervals from 9:00 am to 9:00 pm with an OBF tonometer (OBF Labs, UK, Ltd). The Goldmann intraocular pressure (IOP) and 'erect arm' systemic blood pressure (BP) were also measured. RESULTS: The IOP was found to be higher in the daytime, reaching the highest at noon (mean of 14.29 mmHg) and gradually reduced to the lowest at 9:00 pm (mean of 12.99 mmHg). The change was marginally significant (repeated measures ANOVA, P = 0.05). The POBF demonstrated a trend to increase from 9:00 am, mean of 605.5 &mgr;l/min, to 9:00 pm, 720.1 &mgr;l/min (repeated measures ANOVA, P < 0.01). Student-Newman-Keuls post hoc test indicated that the difference was mainly due to the comparisons between the 9:00 am and 9:00 pm results and the 9:00 am and 6:00 pm results. The pulse amplitude did not vary significantly. The mean blood pressure also demonstrated a significant variation (repeated measures ANOVA, P < 0.01). In the analysis of covariance, no significant effects of mean blood pressure on POBF and pulse amplitude were revealed. CONCLUSIONS: The variation in POBF was due to factors other than systemic blood pressure. Practitioners should consider POBF variation in repeated measurements, for example when monitoring the medical treatment for glaucoma. Further study on POBF variation is required over a complete circadian cycle.