We have defined myasthenia gravis (MG) in the elderly as onset after the age of 50 years. MG is diagnosed more often today than previously. The increase is mainly found in patients over the age of 50 years. Neurologists therefore see more old patients with MG now than before. Prevalence of the early-onset form of MG seems to be unchanged. Recent data indicate that MG may still be substantially underdiagnosed in very old people. Ptosis, diplopia, weakness of the facial muscles, and problems of articulation are important clinical signs in MG and are easier to detect in a youthful appearance. Since ageing causes a decrease in the total eyelid area with sagging of the lower eyelids, a ptosis may be more difficult to diagnose in the elderly. In addition, diplopia may not be detected because of reduced vision due to macular degeneration or cataract formation. Ocular symptoms of MG are therefore more easily missed in the elderly. Thymomatous MG is more common among older patients than it is in younger onset. The mean age at onset of MG for thymoma cases is 50-60 years. Approximately 10-15% of all MG patients have a thymoma, and around 40% of all thymoma cases are associated with MG. During normal aging, the thymus tissue becomes atrophic and replaced with fat. Recent data on MG thymus pathology suggest that lymphocyte accumulation indicating residual thymus may also be found in the elderly, and that there is little qualitative difference between the young and the old thymus from MG patients. The mean concentration of antibodies to acetylcholine receptor (AChR) is lower in MG in the elderly than in early-onset or thymoma-associated MG. Seronegative MG is less common among older patients. Approximately 30% of patients with late-onset, nonthymoma MG have antibodies to titin, while such antibodies are extremely scarce in early-onset MG. Titin antibodies in MG patients seem to be associated with a higher frequency of DR7 antigen and a decrease of DR3 antigen. The antibody response in MG may therefore be influenced by the genetic background.