Connective tissue diseases represent an heterogenous group of immunologically mediated inflammatory disorders including multiorgan involvement, can affect pleura with various frequencies. Rheumatoid arthritis and systemic lupus erythematosus represent the most common immunological diseases that affect the pleural cavity. The clinical presentation, frequency, prognosis and response to therapy vary, depending on the pattern of involvement as well as on the underlying connective tissue diseases. In this article, we will review the pleural effusion caused by connective tissue diseases. (Korean J Med 2011;81:154-160)