AIM: To analyse 24 hour variations in intraocular pressure (IOP) and central corneal thickness (CCT) in a group of glaucomatous patients. METHODS: 30 patients with primary open angle glaucoma were hospitalised and underwent circadian evaluations (at 8 pm, midnight, 4 am, 8 am, noon, and 4 pm) of supine and sitting IOP, respectively, measured using a Perkins and a Goldmann tonometer, and CCT measured using an ultrasonic pachymeter (the mean value of three measurements within 5 mum). All patients were treated with timolol 0.5% twice daily and latanoprost 0.005% once daily. RESULTS: Mean supine IOP was 15.3 (SD 3.7) mm Hg (range 10-25), with circadian fluctuations of 7.3 (3.3) mm Hg. Mean sitting IOP was 15.1 (3.9) mm Hg (range 8-26), with circadian fluctuations of 5.4 (3.1) mm Hg. Mean CCT was 534 (39) microm (range 443-637 microm) with circadian fluctuations of 16.5 (6.2) microm (range 6-31 microm). Both the within patient and within time point fluctuations in CCT were statistically significant (p<0.0001, ANOVA). CONCLUSIONS: The authors found considerable fluctuations in 24 hour IOP. The circadian fluctuations in CCT were small and, although statistically significant, did not seem to interfere with the circadian IOP assessment.