Follicular lymphoma is a B cell malignancy prone to transformation into a large cell diffuse histology. This progression may be multi-clonal as determined by IgH rearrangement. Similar multi-clonal occurrences have been described in immunocompromised patients. However, the lymphoma cells remain predominantly of B cell type. Rarely, composite lymphomas with diffuse T cell histology have been reported arising from follicular lymphoma. The development of T cell leukaemia in a patient with a pre-existing B cell malignancy is an extremely rare event. The occurrence of T cell acute lymphoblastic leukaemia (T-ALL) following follicular lymphoma (FL) has not previously been reported. We report a case of Philadelphia positive (Ph+) T cell ALL developing in a patient who previously had FL which may give some insight into the cell of origin and the defects responsible for malignant transformation of the lymphocytes.