The natural course of autoimmune canine MG was determined in 53 dogs with muscular weakness and a positive acetylcholine receptor antibody titer. Dogs were treated with anticholinesterase therapy, without immunosuppression. Spontaneous clinical and immunologic remission occurred in 47 of 53 dogs within an average of 6.4 months. Neoplasia was identified in the six dogs that did not spontaneously remit. This study questions the value of using canine MG in studies designed to assess the effect of immunotherapies.