To describe characteristics of persons with late (at or after death) acquired immunodeficiency syndrome (AIDS) diagnosis, we analyzed national surveillance data among all persons with AIDS diagnosed through December 1991 under the pre-1993 AIDS case definition and with a known date of death. Late diagnosis was present in 15.8% of 163,202 decreased persons with AIDS and in 15.3% of decreased men with AIDS, 20.6% of women, 12.1% of whites, 20.0% of blacks, 21.1% of Hispanics, 12.3% of men who have sex with men (MSM), 21.9% of injecting drug users (IDU), and 19.6% of persons exposed to human immunodeficiency virus (HIV) through heterosexual contact. When age, race/ethnicity, sex, geographic region, and transmission mode were included in logistic regression analyses, among adults/adolescents, late diagnosis was more likely among persons 40 years or older than among those 13-39 years old, among blacks and Hispanics than among whites, and among IDU and persons exposed to HIV through heterosexual contact than among MSM. Although children (less than 13 years of age) were more likely to have late diagnosis than adults and adolescents, late diagnoses among children did not differ significantly by race/ethnicity, sex, geographic region, or transmission mode. Late AIDS diagnosis, especially among ethnic minorities and IDU and their sex partners, may represent delays in HIV diagnosis and care. In addition to not receiving early clinical intervention, persons who are diagnosed later in the course of HIV disease represent missed opportunities for receiving prevention efforts such as education, counseling, and substance abuse treatment.(ABSTRACT TRUNCATED AT 250 WORDS)