Five patients with clinically overt myocarditis in the setting of systemic lupus erythematosus were analyzed in terms of associated clinical and serologic features. Myositis and antibodies to nuclear ribonucleoprotein (RNP) were present in all. A retrospective review in 140 consecutive patients with systemic lupus erythematosus, including three of these five, showed a highly significant association of myocarditis with myositis (P less than 0.0005). The presence of antibodies to RNP in this small group did not attain statistical significance (P less than or equal to 0.10). The pathologic findings in the one patient who died showed similar patterns of inflammation in both cardiac and skeletal muscle, suggesting the possibility of a generalized inflammatory process directed against striated muscle. Furthermore, although anti-RNP antibodies were found uniformly in these patients, their significance remains to be defined.