In several countries, the prevalence of smoking among schizophrenia patients is extraordinarily high (70% or greater). A State hospital survey demonstrated that after correcting for other factors, inpatients who have schizophrenia are more likely to be smokers than are persons in the normal population or even other chronic psychiatric inpatients. These findings suggest that neuroleptic treatment and the hospital environment cannot completely explain the high prevalence of smoking. The relationship between smoking and schizophrenia may be explained by a combination of three factors. One factor is the great difficulty for schizophrenia patients to quit smoking. Another is late onset: some schizophrenia patients start to smoke after the onset of psychosis. A third is increased early onset: schizophrenia patients may start daily smoking in greater numbers during adolescence (before the onset of their psychosis) than do persons in the normal U.S. population. Daily smoking is usually considered a sign of nicotine addiction and is used by epidemiological surveys to define the prevalence of smokers (most smokers smoke daily and very few smokers do not smoke every day). The increased early onset suggests that familial factors may increase the prevalence of smoking even among patients who have not yet shown psychotic symptoms. It is hypothesized that smoking among family members in families with genetic loading for schizophrenia may be a marker for those at risk of developing schizophrenia.