We describe a new method for analysis of change in glaucomatous visual fields with the object to differentiate between changes caused by glaucoma from those caused by cataract. New pattern deviation change probability maps were developed from a prospectively collected glaucoma material and designed to be sensitive to changes in localized field loss, but to be unaffected by media-induced perimetric change. We compared the new change probability maps with the commercially available total deviation change probability maps in series of Humphrey perimetric tests in a glaucoma material of 43 eyes of 35 patients, who had undergone cataract surgery. When using the total deviation maps, considerable differences were seen between fields obtained before and after cataract surgery. Much smaller differences were seen when using the new change probability maps, that almost eliminated the common and disturbing effect of increasing cataract. This new tool could be of considerable help in differentiation between progressive glaucomatous visual field loss and deterioration caused by increasing media opacities.