OBJECTIVE: To verify if a loss in electrophysiological contrast sensitivity in a group of patients with bifocal posterior chamber intraocular lenses (PC IOLs) can be demonstrated, with respect to monofocal PC IOL-patients and healthy phakic ones. STUDY DESIGN: Prospective study of electrophysiological contrast sensitivity in randomly selected clear lens or pseudophakic patients, over 3 months after surgery. SETTING: Ophthalmic out-patient Palermo University Eye Clinic. PATIENTS: 40 adults divided into four groups of ten eyes each, matched for age, sex, far and near visual acuity, respectively with a two-zone bifocal IOL, a multizone bifocal IOL, a Sinskey-Kratz monofocal IOL, or a natural clear lens. MAIN OUTCOME MEASURES: Mean electrophysiological contrast sensitivity function of each group, obtained from the components CI-CII of the pattern ON-OFF VEPs responses with various spatial frequencies. RESULTS: The two-zone and multizone bifocal IOL-groups exhibit a lower contrast sensitivity when compared with the phakic clear lens group, with all the spatial frequencies tested (P < 0.05). Moreover, they show a lower contrast sensitivity with respect to the monofocal IOL-group with the majority of frequencies (P < 0.05). On the contrary, the latter group shows a lower contrast sensitivity with low-medium spatial= frequencies, but not with higher spatial frequencies, if compared with the phakic group. CONCLUSIONS: Data analysis indicates that while the monofocal IOLs decrease only the low-frequency contrast sensitivity, the bifocal implants depress the whole contrast sensitivity curve to a much lower level. It seems reasonable to hypothesize that, in some particular environmental requirements, this could impair the visual function.