Contemporary alcoholics often use multiple substances, but there is little systematic research on this. This study examines the drug use comorbidity of alcoholics (DSM diagnosis, frequency and 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 data from biological specimens (urine and hair). Lifetime diagnosis of joint alcohol and drug dependence/abuse was 64%. Two-thirds (68%) reported using drugs in the past 90 days: 33% powder cocaine; 29% crack cocaine; 15% heroin, and 24% cannabis. The mean proportions of exposed days on which users reported consuming a substance were 57% (alcohol), 26% (powder cocaine), 46% (crack cocaine), 47% (heroin), and 29% (cannabis). 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.