To assess which factors are associated with the CD4+ lymphocyte count at the time of AIDS diagnosis we studied 3046 patients in the AIDS IN EUROPE study who were diagnosed with AIDS in 1 of 17 European countries between 1979 and 1989 and for whom the CD4 count at AIDS diagnosis was known. Data were extracted retrospectively from patient case notes, using a standardized form. There was a wide range of average CD4+ lymphocyte counts at AID diagnosis, according to which diseases were present at diagnosis. The highest geometric mean CD4+ lymphocyte counts at AIDS diagnosis were associated with the diagnosis of extrapulmonary tuberculosis, Kaposi's sarcoma, and non-Hodgkin's lymphoma while the lowest counts were found when histoplasmosis and cytomegalovirus (CMV) retinitis were present. There were no appreciable differences between CD4+ lymphocyte counts at AIDS in patients according to the three major transmission route categories (sex, age, or region of diagnosis) but there was a marked trend (p < 0.005) toward lower CD4+ lymphocyte counts at AIDS diagnosis in more recent years. These associations remained largely unchanged after adjustment for other factors.