This study was designed to determine whether there are cognitive and memory impairments in patients with myasthenia gravis (MG). Five female patients with MG were tested on two occasions, 4-6 days apart, prior to plasma exchange and after plasma exchange. All subjects had 2-4 separate plasma exchanges. The subjects were tested with mini-mental state examination (MMS), Zung self-rating depression scale (SDS) and a logical memory test. The MG group was significantly impaired compared to the control group on all tests prior to plasma exchange. The MG patients improved in motor function with plasma exchange. In addition, the mean MMS score improved from 26.4 to 28.2 and the mean SDS score improved from 0.46 to 0.39 and the mean immediate logical memory score improved from 5.8 to 8.4 and mean delayed logical memory score improved from 5.8 to 8.0. These improvements were all significant (P < 0.05). The results support the hypothesis that MG has central cholinergic effects, manifested by cognitive dysfunction.