A woman who emigrated to the United States from the Dominican Republic developed the first signs of cutaneous T cell lymphoma during the last trimester of her pregnancy. This patient, found to have a positive reaction against human T-lymphotropic (leukemia-lymphoma) virus type I (HTLV-I), was followed up prospectively from the appearance of the initial skin lesion to the development of high-count helper T cell leukemia. Antibodies reactive with the core protein of HTLV-I were also identified in her husband and mother but not in her 2-year-old daughter. Examination of the patient's course provides clues about the latency period and transmission of HTLV-I and highlights similarities between HTLV-I-positive and HTLV-I-negative cutaneous T cell lymphoma.