Erythrocyte thiamin metabolism and transport were investigated in 7 patients from Brazil, Israel and Italy suffering from thiamin-responsive megaloblastic anaemia (TRMA) associated with diabetes mellitus and sensorineural deafness. All patients discontinued thiamin therapy for 4-7 days before the investigation. TRMA patients showed invariably reduced total thiamin levels in erythrocytes (percentage reduction compared with healthy controls, -46.8+3%; mean+SEM). The proportions of individual thiamin compounds, expressed as a percentage of total thiamin content, were within the normal range, whereas their absolute amounts were significantly decreased in the following order: thiamin monophosphate>thiamin pyrophosphate>thiamin. Thiamin pyrophosphokinase activity was also reduced as compared with controls (mean reduction+SEM, -25.9+1%). The saturable, specific component of thiamin uptake, which normally prevails at physiological concentrations of thiamin (< 2/~mol/L), was absent in erythrocytes obtained from TRMA patients, while the non-saturable (diffusive) component of uptake was normally present.These results confirm observations made previously in two patients and demonstrate that TRMA is consistently associated with a state of thiamin deficiency, which is presumably secondary to reduced thiamin cellular transport and absorption (caused by lack of a membrane-specific carrier), and to impaired intracellular pyrophosphorylation.Investigators from our group recently showed that erythrocytes obtained from patients with thiamin-responsive megaloblastic anaemia (TRMA, McKusick 249270), exhibit no saturable, specific component of thiamin transport as well as a reduced thiamin pyrophosphokinase (EC 2.7.6.2; TPK) activity, resulting in decreased red cell total MS received 22.3.94 Accepted 16.5.94