The analytical program .DELTA. was used to determine long-term fluctuations and measurement accuracy of Octopus program 31 when used with glaucoma patients. Program 31 examined the 30.degree. field. The test locations were arranged in a square grid with 6.degree. resolution. The program .DELTA. determined and compared the disturbed area in %; the total loss, the total sensitivity was around 2000 dB; and the total loss in dB per mean number of disturbed points. Eyes (32) of 22 patients with established glaucomatous field defects were examined twice within 2-6 days and then again 2 mo. later. The sample was classified in 3 groups by the size of the disturbed area: the 1st group had a disturbed area of 1-33%; the 2nd group, disturbed area 34-66%; 3rd group, disturbed area 67-100%. The following long-term fluctuations and accuracy of measurement were determined: disturbed area was between 0.7 .+-. 8% in group 3 and 1.7 .+-. 13% in group 2. The total loss increased proportionately to the disturbed area and was 4.9 .+-. 29.2 dB in group 1 and 31.8 .+-. 82.4 dB in group 3. The total loss per mean number of disturbed points was 0.5 .+-. 2 dB in group 1 and 0.3 .+-. 1.2 dB in group 2. If the learning effect was over, changes of more than 2 dB, especially if several adjacent points were affected, were a significant loss. The learning effect may go up as high as 2 dB/point.