This report describes a case of a female infant of congenital leukemia with a chromosomal translocation t(11;19) (q23;p13) which presented initially with lymphoid features and at relapse with monocytic ones. The clinical course and the results of sequential cytochemical, cytogenetic and immunological studies are considered to be consistent with the interpretation of leukemogenesis of the myelo-monocytoid progenitor cell which still retains the capability of exhibiting lymphoid features to a limited extent. Although leukemia with t(11;19) has been classified as ANLL, most commonly M5 of FAB classification, the patient with this chromosomal abnormality may have a mixed leukemia in which cells with lymphoid features and those with monocytic ones exist.