We have studied the effects of dermatochalasis on Humphrey automated perimetry of the central 24 degrees visual field. Fifteen visual fields of 9 ocular hypertensive patients (18 eyes) were found to be incongruous with their apparently healthy optic discs. Examination revealed dermatochalasis, which was felt to be responsible for the field defects. This was confirmed by reversal of the defects on repeating the field test (programme 24-2) with the redundant upper lid skin taped up, or in 2 cases following blepharoplasty. The defects always involved the superior visual field. The deepest and largest defects were sited in the supero-temporal quadrant in 13 of the 15 affected fields and the supero-nasal quadrant in 2 fields. The most common pattern was a temporally skewed defect which reflected the tendency of the loose upper lid skin to be greater in extent temporally than nasally. In 7 fields the supero-temporal defect extended to fuse with the blind spot, mimicking a superior arcuate scotoma. Temporal extension of the field defects below the horizontal meridian occurred in 5 fields. In cases where visual field testing was repeated without taping up the lid inter-test fluctuation in scotoma size and depth was observed, although the position of scotomas when present within the visual field remained constant. We conclude that dermatochalasis has the potential to confound diagnostic automated visual field testing for glaucoma.