Among injection drug users (IDUs) entering drug treatment in King County, Washington between 1988 and 1991. we investigated HIV seroprevalence in relationship to demographic, sexual, and drug-use characteristics. Eighty-two of 3,039 (2.7%) IDUs tested HIV positive. Gay or bisexual men had the highest HIV prevalence (37.1%), followed by lesbian or bisexual women (8.3%), heterosexual men (2.3%), and heterosexual women (1.5%). American Indians were more likely to be infected with HIV than were whites. Those with no permanent address were more likely to be infected than those with an address. Unexpectedly, the prevalence of HIV infection among amphetamine injectors (13.1% of 168) was higher than among those who did not report using amphetamines. After adjustment for sexual orientation, HIV prevalence was four times higher among primary amphetamine injectors and three times higher among secondary amphetamine injectors than among injectors of other drugs. The basis for the strong association observed between HIV infection and a history of injection of amphetamines is not known and should be clarified through further research that obtains more detailed information on IDUs.