Cone increment thresholds were measured psychophysically in 42 patients (ages 10 to 59) with retinitis pigmentosa and visual acuities of 20/20 to 20/70. A 6-min arc diameter stimulus was flashed to the foveola at a background retinal illuminance for which Weber's law was in effect; under these test conditions, cone increment thresholds depended on the number of remaining functioning cones (cone spatial density) but not on their outer segment length (cone optical density). Log threshold and log visual acuity were correlated inversely (P less than 0.001). The slope of the regression line relating these two measures was compatible with the idea that reduced cone spatial density is a major determinant of reduced visual acuity in retinitis pigmentosa. In a subset of these patients with visual acuities of 20/25 or better, results of cone increment threshold testing suggested that cone spatial density was reduced significantly less in the foveola than in the parafovea.