- ABSTRACT Objective The objective of this study was to define the prevalence of systemic lupus erythematosus (SLE) in patients with myasthenia gravis (MG). Methods Seventy-eight MG patients recruited unselectively from Israeli MG database were evaluated by medical history, physical examination and serology (ANA at 1:100 and anti-ds-DNA at 1:10 dilution) for the presence of SLE, which was defined by the presence of four or more American College of Rheumatology diagnostic criteria. Results Thirty-one (40%) of our patients were males and 47 (60%) were females. Their mean age at time of the study was 51.5 ± 14.5 years. Forty patients (51%) had an early-onset disease (<40 years); 90% had generalized and 10% had limited ophthalmic MG. Significant titers of ANA and ds-DNA autoantibodies were observed in 38.5% and 19.2% of the patients. In six (7.7%), a definitive diagnosis of SLE was established (MG was first diagnosed; there was no association with previous thymectomy), three of them revealed lupus-related neurologic manifestations. All six patients were females with an early onset generalized MG. Conclusion High prevalence of SLE and lupus-related autoantibodies exist in female MG patients. Thus, MG patients should be evaluated for the coexistence of SLE, and assessment for MG is suggested in lupus patients with unexplained muscular weakness.