The Octopus automatic perimeter was used to determine changes in visual field of 90 patients with chronic open angle glaucoma. Mean sensitivity value calculated from 62 of the 73 points of the apparatus program 33 - after elimination of those 11 points corresponding to Mariottes's zone and the central 10 degrees - provides an effective paracentral visual field monitoring numerical parameter. The authors named the parameter "global value". Statistical analysis of "global value" variations between two successive examinations showed: significant improvement between the first and the second tests; disappearance of the phenomenon between two further successive tests. The reasons for this pseudo-improvement remain unclear: perhaps the patients experience gained from the precious tests plays a part in the phenomenon. This should in any case be taken into account when assessing possible aggravation between two tests. Analysis of results suggests that, as a standard evaluation method, any "global value" degradation of the order of 1 db between the first two tests and 3 db between two successive tests performed after the second one is significant of a visual field degradation with a 1% range of error.