The exact relationship between myasthenia gravis (MG) and extrathymic malignancies has not been established thus far. Occasional cases of MG have been reported in association with lymphoma or other lymphoproliferative disorders. To determine the risk of extrathymic malignancy with particular attention on lymphoid malignancy for MG patients in a large cohort representing 99% of the Taiwan population, claims data from the Taiwan National Health Insurance Database were used to conduct retrospective cohort analyses. The study cohort comprised 3671 MG patients who were 10-fold frequency matched by age and sex, and assigned the same index year without MG. Cox proportional hazard regression analysis was conducted to estimate the risk of cancer. The MG cohort had a 1.74-fold increased risk of developing cancer compared to the comparison cohort (HR=1.74, 95% CI=1.47-2.05). After adjusting for confounders and relative to the cohort, patients with MG had a 2.27-fold increased risk of developing lymphoid malignancies (HR=2.27, 95% CI=1.06-4.88) and a 118.47-fold increased risk of thymus cancer (HR=118.47, 95%=42.57-329.71). This population based retrospective case-control study confirms and extends previous observations on the association between MG and lymphoid malignancies.