A case of acute lymphoblastic leukaemia in a 17-year-old male is described. The patient had many distinctive features including a very high blast cell count, prominent lymphadenopathy and hepatosplenomegaly, thymic mass, and a fulminant clinical course. Immunological studies on the blast cells using a variety of techniques showed the presence of two distinct subpopulations, one having the surface characteristics of thymus-dependent (T) lymphoid cells and the other those of bursa-equivalent (B) lymphoid cells. The case therefore represents the first example of a combined T and B cell acute leukaemia.