The results are reported of 38 ultrasonographic in vivo mensurations of intraindividual differences in axial thickness between a cataractous lens in one eye and a biomicroscopically clear or slightly cataractous lens (incipient deep cortical opacity) in the other. Obviously intumescent cataractous lenses were excluded. In general, the cataractous lens was thinner than the contralateral clear or slightly cataractous lens. Large decreases in lens thickness appeared in lenses with the capsule-near opacities of posterior subcapsular cataract (PSC) + anterior capsular/subcapsular opacity (ACSCO). PSC was more closely correlated to lens thinning than was ACSCO. Nuclear cataract very often occurred in thin lenses, but did not appear to cause lens thinning per se. Deep cortical opacity was not associated with lens thinning. The present results contributed to our argumentation that the decrease in lens thickness is due to a leak of lens material through the "lens membrane" beside a possible cessation of growth of the lens fibres.