It has been 10 years since the discovery of the human T-cell lymphotropic virus type I (HTLV-I), the first human retrovirus. During the past decade, significant progress has been made in understanding the transmission of the virus and defining its geographic distribution. It has been shown conclusively that HTLV-I is a causal factor in the induction of both adult T-cell leukemia/lymphoma and HTLV-I-associated myelopathy. However, the pathogenesis of each of these conditions is not clear, and in the light of the evidence of immune dysfunction seen among carriers of the infection, it is likely that other associated diseases will be identified. The challenge in the next decade will be to develop and implement therapeutic interventions among carriers to prevent such diseases as well as to curtail transmission within endemic populations.