The Centers for Disease Control (CDC) has proposed revising the AIDS surveillance definition to include any HIV-seropositive person with a CD4 cell count of less than 200 cells per microliter. Based on a study of persons receiving treatment for HIV infection, this new definition would lead to an estimated 50% increase in the number of persons recognized as living with AIDS. Among 440 HIV-seropositive research subjects recruited from drug treatment programs and through street outreach in New York City, 59 met this definition, yet only 25% of those had been reported to the New York City AIDS registry. The new definition, if combined with HIV and T-cell testing at drug treatment and street outreach programs, could thus yield very large increases in the number of injecting drug users meeting the new surveillance definition of AIDS.