Two patients from Puerto Rico with progressive paraparesis had serum positive for human T lymphotropic virus type I (HTLV-I) antibodies by ELISA and Western blot, and one patient had HTLV-I antibodies in CSF by the ELISA method. Although the Caribbean basin is considered to be an endemic area for tropical spastic paraparesis, this is the first report of the isolation of HTLV-I antibodies in the serum and CSF of patients with chronic myelopathies in Puerto Rico.