Thiamin-responsive megaloblastic anaemia (TRMA, McKusick 249270) is a rare syndrome comprising the triad of profound congenital sensorineural deafness, childhood-onset diabetes mellitus, and a variable anaemia; the last two features often show a measurable improvement with oral thiamin treatment (Rogers et al 1969). The high frequency of consanguinity in affected families suggests autosomal recessive inheritance. The underlying defects are thought to include defective thiamin cellular uptake and pyrophosphorylation (Rindi et al 1994). Although erythrocyte thiamin metabolism has been investigated, muscle chemistry has not been reported; we therefore aimed to investigate two unrelated affected patients.