Substance abuse histories from 1544 men and women receiving treatment at 10 federally funded centers indicated that 80% had abused, either sequentially and/or concurrently, 2 or more substances at some time during their lives. The equivalent figure for the last 3 mo. before being interviewed was 56%. A preponderance of multiple substance abuse, especially of alcohol and various other drugs, was indicated over single substance abuse (SSA). SSA nearly always involved alcohol and was reported mostly by those 40 yr or older. Lines of inquiry for diagnosing alcohol abuse by addicts and drug abuse by alcoholics are proveded.