We studied the influence of cataracts on visual fields in 36 eyes of 36 patients before and after cataract extraction with intraocular lens implantation. Lens opacity was quantified with a slit-lamp system. The influence of cataracts on the visual fields was slightly but statistically significantly larger in the central than in the midperipheral region. Lens opacity had a high correlation with the visual field changes and moderate but still significant correlation with changes in visual acuity. Measurement of stray light back scattered from the cataractous lens allowed prediction of the influence of cataract on the visual field.