Simvastatin, a potent blood lipid reducer, has been suspected of inducing cataract in dogs. Twenty-nine type II hyperlipidemic patients on simvastatin were followed over 2 years at half-year intervals. A parallel group of sex- and age-matched normolipidemics were followed over a half year. Patients and controls had similar nuclear and cortical lens opacities, and water clefts. Vacuole number did not differ significantly between right and left eyes, nor between nasal and temporal parts of the lenses. General vacuole index (total vacuole number in a patient) for left eyes was analysed further. Controls showed significantly higher indices than patients, for no obvious reason. Indices for controls increased significantly over the half-year test period, and among patients over each half-year interval. The speed of this increase did not differ significantly from that of the controls. Furthermore, the increase was judged to be clinically insignificant. No deleterious effect of simvastatin on the lens could be established.