Second-degree atrioventricular (AV) block had not been reported as an early manifestation of adult systemic lupus erythematosus (SLE). An 18-year-old woman of SLE presented with asymptomatic second-degree AV block with 2:1 conduction block on electrocardiogram (ECG) during admission. Serologic tests were negative for anti-Sjögren's syndrome A (anti-SS-A/Ro) and anti-SS-B/La antibodies, but positive for anti-ribonuclearprotein antibodies. Her abnormal ECG completely resolved soon after high-dose intravenous methylprednisolone infusion, and she was maintained successfully with a low dose of oral steroid. The possible pathogenesis of this complication is discussed. Follow-up with periodical ECG is recommended for adult lupus patients to screen for possible conduction system involvement, and treatment should be started as soon as possible.