Flow cytometry is the preferred method of diagnosing and immunophenotyping acute lymphoblastic leukemia (ALL). However, there are situations in which immunohistochemical staining (IH) of bone marrow trephine biopsy specimens can be used to provide immunophenotypic information. To evaluate the use of IH and to confirm its value in diagnosing and typing of ALL, we studied 50 cases of denovo ALL that were previously classified into pre B, T and B by morphologic, cytochemical and FC methods. Paraffin embedded bone marrow trephine biopsies sections were stained using a panel of antibodies,namely, myeloperoxidase (MPO), terminal deoxynucleotidyl transferase (TdT), CD10, CD20, CD79a, CD3. The cases included 37 pre BALL, 10 T ALL and 3 mature BALL. TdT was the most commonly expressed antibody and was positive in 41 of 50 cases of ALL (82%) and in 95% of pre B ALL cases. CD79a and CD10 were positive in 68% and 65% of pre B ALL cases, respectively. CD79a showed similar positivity in B ALL cases (66%). CD 20 was positive in 66% of mature B ALL cases but less positive in pre B ALL (22%). CD3 was positive in 70% of T ALL cases and negative in other ALL subtypes. All of the cases were negative for MPO. Diagnosis and immunophenotyping of acute lymphoblastic leukemia is possible using immunohistochemical staining of bone marrow trephine biopsies.