Spontaneous T-cell leukemia similar to human adult T-cell leukemia (ATL) was found in an African green monkey naturally infected with simian retrovirus closely related to human T-cell leukemia virus type I (HTLV-I). Monoclonal integration of the simian retrovirus was detected in the primary leukemic cells, suggesting an association of the retrovirus with ATL-like leukemia in the monkey.