A total of 1,066 serum samples from 911 individuals with repeatedly reactive enzyme-linked immunosorbent assay (ELISA) for antibody to the human immunodeficiency virus type 1 (HIV-1) were enrolled for confirmatory HIV-1 infection diagnosis during the three years from 1990 to 1993. According to the interpretation criteria for the anti-HIV Western blot test recommended by the Centers for Disease Control, 38 (4.2%) were Western blot-positive, 110 (12.1%) were Western blot-negative, and 763 (83.7%) were Western blot-indeterminate. The most common band patterns of indeterminate Western blot results were antibodies to gag gene product only (667/763, 87.5%) which included p18 only (180, 23.6%), p18 plus others (521, 68.3%), p25 only (55, 7.2%), and p25 plus others (212, 27.8%). Eighty-three individuals with indeterminate Western blot results were followed-up and new serum samples were collected. None of the follow-up samples became positive. When band patterns changed, they usually did so within the specific category (either gag, pol, or env), such as a change from p18 to its precursor p55. All of the indeterminate specimens tested by particle agglutination assay showed negative reaction. In conclusion, an indeterminate result should not been seen as final; laboratory testing is required on the follow-up specimens.