The study examined the drug use comorbidity of alcoholics (DSM diagnosis, frequency, quantity of drug use); the relationship between drinking and drug use; the relative severity of alcohol- and drug-related problems; and the validity of reports of illicit drug use. Data on substance use were collected from 248 treatment-seeking alcoholics using an expanded Time-line Follow-Back (TLFB) interview. Self-reports of substance use were validated with urine and hair samples. Lifetime diagnosis of joint alcohol and drug dependence/abuse was 64 percent. Subjects reported consuming an average of 14 standard drinks on a drinking day and $67 worth of drugs on a using day. Drug users reported drinking less than nonusers on a drinking day. Frequency of drinking and drug use were positively correlated; almost all drug users reported simultaneous drinking and drug use, and they rated drugs as the bigger problem. Considerable under-reporting of drug use occurred for the previous 3-4 days, but was more accurate for the previous month. Tables, notes, references