The Noonan syndrome (NS) is a multiple congenital anomalies (MCA) syndrome with well-known manifestations. Excessive bleeding has been described occasionally. We report on 19 patients with NS and a bleeding diathesis. Several different defects are identified in the coagulation and platelet systems occurring singly or in combination. Clinical expression is variable. It is concluded that bleeding diatheses occur in NS at a much higher frequency than previously suspected. Consideration is given to possible relationship to underlying metabolic defects which could explain the diverse nature of the bleeding diatheses and also play a role in the pathogenesis of NS. The variety of bleeding diatheses may also reflect heterogeneity within NS. NS patients frequently undergo surgery with increased risk of bleeding. Appropriate evaluation and management is discussed. Evaluation of all NS patients and their families for bleeding disorders should provide important information about the frequency and type of bleeding diatheses which occur and perhaps help to clarify the etiology and pathogenesis of NS.