To study the clinical features and immunological changes in patients of childhood-onset myasthenia gravis (CMG). Methods: Seventy randomly selected CMG patients and 100 age-matched healthy controls were recruited. Clinical data of the CMG group such as onset age, MG classification, gender, comorbidities, and thymus abnormalities were collected to summarize the clinical features of CMG. Peripheral blood was collected from subjects in both groups. The CMG group serum AChRAb levels were detected. Immunological testing (IgG, IgA, IgM, C3, C4) was performed on both groups. The concentrations of IgG subclasses in both groups were quantified using Quantibody Human Ig Isotype Array. Results: Of the CGM patients in this study, approximately half had an onset age before 5 years old, and a majority presented with ocular type MG (92.9% ). Comorbidities of thyroid abnormality were present in 20.0% of patients, and comorbidities of thymic abnormality were present in 14.3% of patients; 67.1% of patients were positive for AChRAb. Compared with the controls, the concentrations of serum IgG, IgA, IgM, C3, and C4 in CMG patients were significantly reduced (P<0.05). Among the IgG subclasses, IgG1 and IgG3 concentrations were higher in CMG patients than in healthy controls (P<0.05); the difference in other IgG subclasses was not significant (P> 0.05). Conclusion: Our data show that a majority of CMG patients develop the disorder before 5 years of age, present with ocular type MG, and show a high positive rate of autoantibodies. Our results suggest that AChRAb is mainly composed of IgG1 and IgG3 and that CMG is characterized by decreased levels of immunoglobulins and complements