PURPOSE. To develop a sensitive psychophysical test for detecting visual defects such as microscotomas. METHODS. Frequency-of-seeing curves were measured with 0.75′ and 7.5′ spots. On each trial, from 0 to 4 stimuli were randomly presented at any of eight equally spaced loci 0.5° from fixation. By correcting the aberrations of the eye, adaptive optics produced retinal images of the 0.75′ spot that were 3.0 μm wide at half height, small enough to be almost entirely confined within the typical cone diameter at this eccentricity. Data were collected from a patient with deuteranopia (AOS1) whose retina, imaged with adaptive optics, suggested that ≈30% of his cones were missing or abnormal. Patients with protanomalous trichromacy (1 subject), deuteranopia (1 subject), and trichromacy (5 subjects) served as controls (all had normal cone density and complete cone mosaics). Psychophysical results were modeled by a Monte Carlo simulation incorporating measured properties of the cone mosaic. RESULTS. Frequency-of-seeing curves for AOS1 obtained with 0.75′ spots showed lower asymptote, slope, and sensitivity than for controls. The 7.5′ results showed that these differences were the result of the small spot size, which on some trials was confined mostly to the locus of the putatively missing cones. A two-parameter model satisfactorily described the data and was highly sensitive to the proportion of missing cones simulated. CONCLUSIONS. Adaptive-optics microperimetry is a powerful psychophysical test for assessing the loss of neural elements, even in retinas that appear otherwise normal in standard clinical tests. This technique may prove useful in estimating the proportion of missing cones in different patients and in detecting other visual losses such as those associated with glaucoma. Copyright © Association for Research in Vision and Ophthalmology.