Cyclic adenosine 3',5'-monophosphate (cAMP) signal transduction was examined in lymphocytes and platelets obtained from patients with posttraumatic stress disorder. Intact lymphocytes from the posttraumatic patients (N = 10) showed significantly lower basal, isoproterenol-, and forskolin-stimulated cAMP levels than those from 10 healthy control subjects. In platelet membrane preparations, basal, forskolin-, aluminum chloride plus sodium fluoride-, and prostaglandin E1-stimulated adenylate cyclase activity levels were all significantly lower in the posttraumatic group than in the control group. The authors discuss the potential role of their findings as a biological marker for posttraumatic stress disorder.