Hairy cell leukaemia occurring during phenytoin (diphenylhydantoin) treatment

We describe the case of a patient who developed hairy cell leukaemia (leukaemic reticuloendotheliosis) during phenytoin treatment. Hairy cells were identified by fluorescence, phase contrast and electron microscopy; they contained tartrate-resistant acid phosphatase activity, formed rosettes with mouse but not sheep erythrocytes and bore monoclonal surface immunoglobulin. Because of the association of pseudo- and true lymphomas with phenytoin it is possible that this lymphoproliferative disorder arose as a result of the treatment with phenytoin, possibly in conjunction with sulthiame.