This study was conducted to measure the impact of laboratory-initiated reporting of CD4+ results on reporting of AIDS in the United States. States were categorized by whether CD4+ reporting was required; we compared the number and percentage of AIDS cases reported based on immunologic criteria, controlling for whether states also required HIV infection reporting. We observed cases reported in 1994 with CD4+ values and the delay between diagnosis and report by CD4+ and HIV-reporting status. From 1992 to 1994, states with CD4+ reporting had a greater proportionate increase in reported AIDS cases (98%) than states without CD4+ reporting (55%; p < 0.0001). From 1993 to 1994, the eight states with both CD4+ and HIV reporting had a higher increase in cases meeting immunologic criteria (7%) than the 13 states with only HIV reporting (< 1%), the three states with only CD4+ reporting (< 1%), and the 16 states with neither form of laboratory reporting (4%). Of 1987 definition cases reported in 1994, the percentage reported with CD4+ values was lower in states without either CD4+ or HIV reporting (79%) than in states with both CD4+ and HIV reporting (83%), only HIV reporting (84%), or only CD4+ reporting (88%). The percentage of AIDS cases reported within 3 months of diagnosis was lower in states without laboratory reporting (40%) than in states with CD4+ reporting (45%, p = 0.001). CD4+ reporting may enable states to report AIDS cases earlier in the course of HIV disease, permitting early targeting of health care and social services.