We recently treated two narcotic addicts with bacterial endocarditis who developed the syndrome of inappropriate antidiuretic hormone secretion (SIADH). This prompted a retrospective review of blood chemistry studies in all narcotic addicts admitted to our hospital over a 30-month period because of a clinical suspicion of bacterial endocarditis. Patients with culture-positive endocarditis (group 1) had significantly lower plasma osmolality, sodium, calcium and albumin values (P<.02, .001, .005, and .005 respectively) than addicts without endocarditis (group 2). More than 90% of those in group 1 had hyponatremia, and 48% had plasma hypoosmolality. These findings may be of value in the initial evaluation of ill narcotic addicts for hospitalization.