INTRODUCTION: During the last two decades, bone marrow examination has become an indispensable adjuvant to diagnose the malignant diseases of the blood and other body systems. The ease with which a marrow trephine biopsy can now be combined with aspiration, it is suggested that this can be used more frequently with suspected neoplastic diseases. This will obviate the need for many other expensive and time-consuming investigations. AIMS & OBJECTIVES: We decided to study the comparison of relative efficacy of bone marrow aspiration and trephine biopsy, and cytological and histological features in different haematological and non-haematological malignancies. METHODOLOGY: Total 52 patients with clinical suspicion or diagnosis of any haematological or non-haematological malignancies were studied from March 2011 to Feb 2012. Patients were enquired about their presenting complaints along with clinical course and duration, and past illnesses. Aspiration smears were examined for cellularity, M: E ratio, megakaryocytes, erythropoiesis, myelopoiesis, plasma cells and abnormal cells. Trephine was also examined for trabecular structure; intertrabecular spaces; cellularity; number, distribution pattern and morphology of megakaryocytes; myelopoiesis; erythropoiesis; fibre content; and lymphoid tissue morphology and distribution. Imprint smears aided cytomorphological characterization of immature cells, observing the pattern and reticulin grading from 0 to IV. OBSERVATION AND RESULTS: Of the total 52 cases 67.3% (35) were haematological and 32.7% (17) non-haematological malignancy cases. Of these 11.5% were AML, 23.0% ALL, 13.5% CML, 7.7% CLL, 5.8% NHL, 1.9% MDS, 3.9% MM, 21.2% CAB, 7.7% CAP, and 3.9% CAS. 75% of 12 ALL cases were in between 11-20 years and M: F was 11:1. Majority had anemia and fever; and 91.6% were classified as ALL-L-2 and the remaining as ALL-L-1. 16.6% cases had 'dry tap' and were diagnosed on imprints. 42.9% of 7 CML cases belonged to 31-40 years and M: F was 6:1. All had anemia and splenomegaly; and 28.5% yielded 'dry tap' and diagnosed on trephine and cellularity & topographic relationship was also better appreciated. 50% of our 4 CLL cases belonged to 61-70 years and M: F was 3: 1. All had anemia as the only clinical presentation. Trephine was done only in 3. In MM 50% belonged to 61-70 years while the other 50% belonged to 71-80 years and both were male. Both had bone pain and 50% also had fracture hip. Rouleaux formation was seen in one while other showed leukoerythroblastic peripheral blood picture. One was classified as plasmacytic type while the other as plasmablastic type. Both aspiration and trephine was done in all non-haematological malignancy cases and bone marrow was normoblastic normocellular. 45.4% showed fibrosis in CAB of which 60% were grade II. Similarly 75% showed fibrosis in CAP of which 33% were grade II. Only one case of CAS showed grade III fibrosis. The 'test of proportion' performed between success rates of Trephine biopsy and Aspiration provided a significant Z value of 2.0008 (Z>1.96 and p <= 0.05). CONCLUSION: It is concluded that Aspiration is the most effective method for studying cellular morphology and detection of leukemias, MDS and identification and classification of lymphomas. Imprints however facilitate the study of cellular details; assessment of marrow infiltration and aid in 'dry tap'/scanty material when aspirate cannot be obtained because of malignant infiltration of marrow. Although trephine sections provide maximum information, all three procedures were found complementary to each other and should be evaluated simultaneously for complete bone marrow interpretation.