A rare variant of non-Hodgkin's lymphoma, signet ring lymphoma of T cell phenotype (only the fourth to be reported) in a 75 year old man was studied by light microscopy, immunohistochemistry, electron microscopy and gene rearrangement studies. Ultrastructurally, a wider spectrum of cell size and nuclear shape was observed in this case than in the previously recorded cases. The morphology of the signet ring vacuoles was identical to that found in the commoner B cell signet ring lymphoma of clear vacuole type, and it is suggested that the vacuoles derive from multivesicular bodies. The four cases reported so far have all presented with skin disease, and the limited evidence available suggests that the prognosis may be good.