Substance abuse histories from 1,544 men and women receiving treatment at 10 federally funded centers indicated that 80% had abused, either sequentially and/or concurrently, two or more substances at some time during their lives. The equivalent figure for the last 3 months before being interviewed was 56%. These data indicated a preponderance of multiple substance abuse, especially of alcohol and various other drugs, over single substance abuse (SSA). SSA nearly always involved alcohol and was reported mostly by those 40 years or older. Lines of inquiry for diagnosing alcohol abuse by addicts and drug abuse by alcoholics are provided.