Masking of the macrocytic expression of megaloblastic anaemia (MA) by coexisting thalassaemia, iron deficiency and chronic illness has been widely reported. We described the haematological and clinical features of 20 Chinese patients with MA presenting with mean corpuscular volume (MCV) < or =99 fl, and analysed the steps leading to the final diagnosis of MA with concomitant thalassaemia trait (n = 11), thalassaemia trait and iron deficiency (n = 3), iron deficiency (n = 4) and chronic illness (n = 2). We also compared the haematological characteristics of this group of patients with a group of normocytic anaemic patients without vitamin B(12)/folate deficiency, and identified certain laboratory information useful for differentiating the two groups. Statistically significant parameters included the mean values of haemoglobin, MCV, red cell distribution width (RDW), reticulocyte index, platelet count and serum bilirubin. All provided clues to maturation disorders within the marrow. A decision flowchart for the diagnosis of MA without macrocytosis was proposed. In the studied population, by using the parameters of haemoglobin <10 g/dl, MCV 80-99 fl, RDW > or = 16% and reticulocyte index < or = 2% as indicators, there was a 58% chance that a patient had MA without macrocytosis if he/she had all the four indicators, and a 2.2% chance of having it if he/she did not have these indicators. We emphasized the importance of including peripheral blood smear examination in the diagnostic procedures for such patients, as well as the importance of paying attention to patients' medical history, racial background and previous MCV value.