We describe a case of human T-lymphotropic virus type I (HTLV-I)-associated transient benign immature T-cell lymphocytosis in a black female patient, which over the course of several months underwent spontaneous complete remission. The patient presented with a white blood cell count of 20,000/microliter and a T4/T8 ratio of 1.7:1. The majority of cells appeared to be lymphoid in origin, and cell marker analyses established that the circulating lymphocytes were predominantly immature T-cells. HTLV-I was detected at this time by a p19 indirect immunofluorescent slide assay. Over a 1-month period of time the patient's clinical status evolved into a mature T-lymphocytosis with a T4/T8 ratio of 4.5:1. HTLV-I was detected by anti-p19 immunofluorescence by cell sorter analyses and by dot-bloc nucleic acid hybridization. Serological testing demonstrated that the patient had anti-HTLV-I antibodies and antimembrane antibodies specific for an HTLV-I producing cell line. In a competitive HTLV-I ELISA assay only HTLV-I proteins could effectively compete out the seroreactivity. The patient also had a high serum level of soluble interleukin-2 (IL-2) receptors, which is associated with HTLV-I infection. This is the first reported case of immature T-lymphocytosis in a patient infected with HTLV-I. The patient's HTLV-I markers disappeared with time, and her lymphocytosis subsequently spontaneously resolved. She remains disease free and virus negative after 2 years of follow-up study.