To study the effect of the intraocular pressure (IOP) on the visual field (VF) damage in low tension glaucoma (LTG), we compared the IOP and the VF damage in both eyes of 46 LTG patients. As an index of the VF damage, we used the mean deviation (MD) value calculated by the Humphry (30-2) STATPAC program, and for the IOP data, the mean of 24 hour IOP measurements (diurnal IOP) and the mean of the IOPs measured at each visit to our outpatient clinics (follow-up IOP). In 13 cases, the diurnal and the follow-up IOPs were 0.3 mmHg higher or more in the more damaged eye than those in the less damaged eye, and in 7 cases they were 0.3 mmHg higher or more in the less damaged eye than those in the more damaged eye. Between these two groups, no significant difference was seen in the value of MD, but the diurnal IOP and the follow-up IOP was significantly higher in the former than in the latter group. By discrimination analysis, the two groups were distinguished at the diurnal IOP of 14.1 mmHg (hit rate 77.6%) and the follow-up IOP of 15.0 mmHg (74.2%). An analysis using the whole 46 LTG cases yielded the following results: when the diurnal or follow-up IOP of the more damaged eye was higher than the above determined level, the IOP in the more damaged eye was significantly higher than that in the contralateral less damaged eye.(ABSTRACT TRUNCATED AT 250 WORDS)