Down syndrome associated with systemic lupus erythematosus: a mere coincidence or a significant association?

An 8-year-old male, who had Down syndrome associated with systemic lupus erythematosus (SLE), is described. He also had a partial complement 4 deficiency. This case is a reminder that the physician should be aware of the possibility of an immune defect in a male presenting with SLE at a young age. The question of whether the association of Down syndrome with SLE is coincidental or whether there is a predilection for autoimmune disorders in Down syndrome is discussed.