Visual Evoked Potentials (VEP) were measured in 9 new-diagnosed hypothyroid female patients--mean age 46 +/- 12 ys--before treatment, during (with monthly evaluations) thyroid hormone replacement therapy and after long-term therapy, at the achievement as well as one year after having achieved and maintained euthyroidism. Three of the hypothyroids had abnormally prolonged latencies (m.v. 131.7 +/- 7.9 ms), while 7 had lower than normal amplitude (m.v. 2.3 +/- 2.8 microV). No remarkable change of amplitude was observed after the achievement of euthyroidism, after a mean time of 5.9 +/- 4.9 months (range 2-14 months). A significant shortening of latency (m 128.3 +/- 7.6 ms), even still higher than the control value (m 122.7 +/- 3.7 ms) was found. Significant correlation between P100 latency and thyroid hormone levels was found for TT4 (r = 0.3353; p = 0.005), TT3 (r = 0.2568; p = 0.032) and FT4 (r = 0.3572; p = 0.002). No further improvement in P100 latency (m 129.5 +/- 7.2 ms; p = 0.037) was found one year after the achievement of euthyroidism, while a remarkable amplitude increase (m 9.2 +/- 3.4 micro; p = 0.001) was observed. Our findings indicate that, as well as other studied parameters, VEP are reversibly alterated in hypothyroidism, probably in relation to metabolic rather than to structural alterations. Moreover, VEP can represent a useful neurophysiologic parameter for quantitation of SNC involvement in hypothyroidism.