Treatment studies suggest that gambling-related irrational beliefs and attitudes (i.e., cognitive distortions (CDs)) contribute to the risk for problem gambling behavior. In a community sample of men, we investigated the associations among lifetime gambling-related CDs, psychiatric disorders other than pathological gambling , and problem gambling severity. Subjects were 1354 members of the Vietnam Era Twin Registry. Problem gambling and gambling-related CDs were derived from a 2002 interview using the National Opinion Research Center DSM-IV Screen for Gambling Problems (NODS). Exploratory factor analysis was performed with the 12 CD items to identify an underlying construct. Generalized linear models were computed to test for associations among CDs, psychiatric disorders other than pathological gambling, and gambling problem severity. Co-twin control analyses of monozygotic twin pairs discordant for problem gambling severity adjusted for genetic and shared environmental influences. Twelve CD items related to one underlying CD construct. After adjustment for lifetime psychiatric disorders, pathological gambling symptoms were positively associated with higher CD scores. Pathological gambling symptoms remained significantly associated with CD scores after controlling for genetic and shared environmental influence. These results provide empirical support for an association between gambling-related CDs and gambling problem severity, even after controlling for genetic and shared environmental influences and non-pathological gambling psychiatric disorders. Public health messages and therapeutic interventions that reinforce the randomness of gambling and draw attention to distorted thinking may prevent the development of problem gambling and improve treatment outcomes.