A manic-depressive patient who developed dysarthria and ataxia while on L maintenance was examined. These symptoms were erroneously attributed to L toxicity occurring at therapeutic serum levels. The symptoms persisted despite diminution in L dosage and a neurological consultation revealed the diagnosis of multiple sclerosis. Lithium therapy is apparently effective with manic-depressive patients, adverse side-effects are infrequent and can occur at toxic or therapeutic serum L levels and the exceptional patient may have a 2nd illness incorrectly attributed to L toxicity.