OBJECTIVE The purpose of the present study was to elucidate the effects of several anti-glaucoma medications on the circadian intraocular pressure (IOP) fluctuations in patients with primary open-angle glaucoma (POAG). METHODS POAG patients (n=61; 61 eyes) with or without glaucoma medications were included. IOP measurement at 14 time points (12, 15, 18, 21, 0, 6, 9, 12, 15, 18, 21, 0, 6, and 9 o'clock) was performed over a period of 48 h. IOP changes occurring in the first 24 h and the subsequent 24 h were evaluated by several therapeutic factors. RESULTS A nocturnal acrophase pattern was observed in all the eyes with POAG. The shape of the first 24 h IOP curve was similar to that of the following 24 h IOP curves. However, there were fewer overall IOP levels in the second 24 h time period. Circadian IOP fluctuation patterns exhibited in each eye on the 1st and 2nd days were single acrophase patterns: diurnal acrophase (1st day, 54.0%; 2nd day, 60.7%) and nocturnal acrophase (1st day, 36.1%; 2nd day, 31.1%), and no single acrophase patterns: flat (1st day, 6.6%; 2nd day, 4.9%) and double acrophase (1st day, 3.3%; 2nd day, 3.3%). Among the different medication groups, a nocturnal acrophase circadian pattern was observed in the patient groups being treated by combinations of prostaglandin analog (PG) and β blocker or PG, β blocker and carbonic anhydrase inhibitor (CAI). However, this was not apparent in patient groups with or without single anti-glaucoma medications or a combination of PG and CAI. CONCLUSIONS The present study of IOP monitoring patients with POAG over a period of 48 h indicated that their changes in circadian patterns of IOP were affected by types of anti-glaucoma medications.