BACKGROUND: The recent recognition of idiopathic CD4+ T-lymphocytopenia (ICL) had led to concern that an unknown immunodeficiency virus may be transmissible by transfusion. STUDY DESIGN AND METHODS: To evaluate the prevalence and significance of low CD4+ values among blood donors, CD4+ data on 2030 blood donors who were negative for antibody to human immunodeficiency virus type 1 (HIV-1) were compiled. Those with CD4+ values below ICL cutoffs (< 300 CD4+ T cells/microL, or < 20% CD4+ T cells) were recalled for follow-up investigations. Serial CD4+ data on 55 homosexual men who seroconverted during prospective follow-up and data on 139 anti-HIV-1-positive blood donors initially evaluated in 1986 were reviewed as well. RESULTS: Five seronegative donors (0.25%) had absolute CD4+ counts < 300 cells per microL and/or < 20 percent. On follow-up, all five donors had immunologic findings within normal ranges, lacked HIV risk factors, and tested negative for HIV types 1 and 2 and human T-lymphotropic virus type I and II infections by antibody and polymerase chain reaction assays. Four of five donors reported transient illness shortly after their low CD4+ count donations. The median interval from HIV-1 seroconversion to an initial CD4+ value below ICL CD4+ cutoffs was 63 months for infected homosexual men. Of 139 HIV-1-infected blood donors studied 1 to 2 years after seropositive donations, 34 (24%) had CD4+ counts < 300 cells per microL and/or < 20 percent. CONCLUSION: Low CD4+ counts are rare among anti-HIV-1-negative volunteer blood donors and are generally associated with transient illnesses. If any unknown virus progresses similarly to HIV-1, CD4+ count donor screening would be a poor surrogate for its detection.