We describe a 77-year-old man with multiple myeloma (MM) who developed Burkitt type leukaemia (BTL) 16 months after the initial diagnosis of MM. MM was positive for CD10 with immunoglobulin (Ig) kappa, kappa Bence Jones protein (BJP) and a normal karyotype. BTL was positive for CD10, CD19, CD20 and HLA-DR with Ig mu and lambda, lambda BJP and a clonal abnormal karyotype of 46,XY,-13, t(8;14)(q24;q32),11q+,14q+, +mar. The patient died 9 d after diagnosis of BTL despite treatment with multiple agents. This is, to our knowledge, the first such case to be reported.