Myasthenia gravis (MG) in individuals over 40 years of age comprises three groups: early-onset MG with prolonged disease duration, late-onset MG with thymoma, and late-onset of nonthymomatous MG. The clinical features do not differ between the three groups, except that early-onset patients with prolonged disease duration usually have a less severe disease. More than 60% of our MG patients are now more than 50 years of age, often with disease onset after age 40. Although 2 out of the 3 patients in Erb's original description had onset of myasthenic symptoms after age 40, this was apparently infrequent in 1879, when the disease was first identified. Onset of MG after age 40 is now common. For example, in our material, 88/184 (47.8%) had onset of MG after age 40. Eighteen (20.5%) had a thymoma. The female:male ratio in the early-onset group was 2.4:1, whereas it was 1:1.1 among those with onset after age 40. There was no human leukocyte antigen association for MG with thymoma. Antibodies to the acetylcholine receptor were detected in 88% of sera from nonthymomatous MG and in 100% of those with late-onset MG with thymoma. Antibodies to titin were found in sera from 85.7% of MG patients with thymoma (all age groups) and in 58% of nonthymomatous MG with late onset and acetylcholine receptor antibodies. Late-onset, nonthymomatous MG comprises two subgroups, one corresponding to delayed early onset and one immunologically similar to that seen in patients with MG and thymoma.