In 47 patients with Parkinson's disease, we studied the visual evoked potential (VEP) using the pattern-reversal checkerboard (CBVEP) and the grating (GVEP) stimuli before and after 3 months of L-dopa therapy. The P100 latency in both CBVEP and GVEP was prolonged before L-dopa therapy, usually commensurate with the degree of motor disability. Clinical improvement of the parkinsonian state after L-dopa therapy was associated with shortening of the P100 latency in CBVEP, but not significantly in GVEP.