OBJECTIVE: To screen for dopaminergic abnormalities in tobacco smokers and patients with posttraumatic stress disorder (PTSD), the authors determined serial CSF and plasma concentrations of the dopamine metabolite homovanillic acid (HVA). METHOD: Continuous subarachnoid sampling was used to obtain 37 serial CSF samples over 6 hours in 13 normal volunteers and 11 patients with combat-related PTSD; 10 smoked and 14 had never smoked. The smokers were abstinent from tobacco for 11 to 17 hours. RESULTS: The smokers had markedly lower CSF, but not plasma, HVA levels. Their CSF HVA concentrations averaged only 54% of the concentrations of the nonsmokers, independent of diagnosis. CONCLUSIONS: Smokers’ low CSF concentrations of HVA may be associated either with chronic inhalation of nicotine or other constituents of tobacco smoke or with acute abstinence. Any possible basal dopaminergic abnormalities in patients with PTSD are small relative to the abnormalities associated with smoking.