Patients in an alcoholism and drug dependency treatment facility were questioned about their gambling behavior in order to find out what percentage of them were abusing alcohol and/or drugs and gambling. In order to do this, a pathological gambling signs index was constructed according to a modification of DSM III criteria and validated using independent procedures. Out of 458 patients interviewed, 40 (9%) were diagnosed as pathological gamblers and an additional 47 (10%) showed signs of problematic gambling. These patients showed clear signs of emotional, financial, family and occupational disruption, and illegal behavior in connection with their gambling which compound the disruption induced by alcohol and/or drugs. Five per cent of the patients abusing only alcohol, 12% of those with alcohol and another drug in combination, and 18% of those with other drug abuse problems without an alcohol component showed clear signs of pathological gambling; 11 1/2% of males and 2% of females were classified as pathological gamblers. The index was also significantly associated with parental gambling (38% of the children of pathological gamblers were pathological gamblers themselves). Gambling by siblings, alcoholism in the father (but not in the mother), gambling prior to age 20, greater amounts of gambling for more money, and "chasing losses in order to get even" were also positively correlated with the index. The implications for study and treatment of these individuals are discussed in terms of the study of addictions.