Thiamine-responsive megaloblastic anemia (TRMA) is an autosomal recessive disorder characterized by the development of megaloblastic anemia, diabetes mellitus, and sensorineural deafness. We report on the first two Croatian patients with TRMA, compound heterozygotes for nonsense, c.373C > T; p.(Gln125Ter) and novel missense variant, c.1214C > G; p.(Thr405Arg) in gene. The first was diagnosed at 4 months with diabetes mellitus and severe anemia requiring transfusions. As TRMA was suspected, thiamine therapy was immediately started to prevent further transfusions and insulin therapy. His brother developed extreme anemia at 3 weeks of age while waiting for the results of the genetic test. Severe anemia in this sibling may have been prevented if thiamine had been initiated earlier.