Atonic pupil: an unusual complication of cataract surgery. Eight cases of atonic pupil, an unusual complication following cataract surgery, are presented. Six of the eyes involved had intracapsular cataract extraction and anterior chamber intraocular lens implantation; one case underwent extracapsular cataract extraction with a posterior chamber intraocular lens implantation; the other had an extracapsular procedure with no implant. The etiology is unknown, but we believe the most likely one is that the sodium hyaluronate used in all these patients caused a temporary reduction of the iris blood supply by mechanical factors or by a vasoconstriction induced by some toxicity of the material that produced a lesion on the iris sphincter. 