Chronic pleural effusion occurred in three patients, one of whom also developed acute pericarditis. A fourth patient developed both pleural and pericardial effusions. All patients had been receiving dantrolene sodium for at least two months. The pleural fluid was a sterile exudate with pleural and peripheral blood eosinophilia in all patients. No pulmonary parenchymal involvement was apparent. Pleural biopsy specimens showed nonspecific inflammation. Resolution of the pleural process was prolonged after dantrolene therapy was discontinued. Although a causal relationship between dantrolene and serosal inflammation remains unproved, this association in four patients warrants careful observation of others receiving long-term dantrolene therapy.