This report describes the occurrence of acute transient myopia in a patient treated with sulfonamide. We followed this patient by performing A-scan ultrasonographic ocular measurements documenting the anterior chamber depth, lens thickness, and axial length during the acute and convalescent periods. The outstanding feature in this case was the documented ultrasonographically significant reduction of the anterior chamber depth combined with lens thickening. This could be caused by a forward displacement of the lens as a result of allergic ciliary body edema and rotation. These changes could explain the mechanism of drug-induced transient myopia.