Acute lymphoblastic leukaemia (ALL) immmunophenotypes were analysed by flow cytometry in 65 Omani patients (46 children, 19 adults). Common ("CALLA-positive") ALL was the most frequently encountered (70%) B-cell lineage immunophenotype. Among T-cell lineage ALL patients, mature T-cell ALL was the least frequent (7%). Expression of certain surface markers including CD20 and CD6 appears to have an effect on some clinical and haematological features but FAB morphology was not useful in predicting cell lineage immunophenotypes. Other interesting findings, currently of uncertain significance, include the sizeable proportions of pre-B-ALL (group V) and cortical thymocytic ALL (stage III). Results of this study should help strengthen the emerging leukaemia database of the recently established Oman National Cancer Register and thereby contribute to a successful global attack against the haematological malignancies.