A child with biotin-non-responsive propionic acidaemia treated with a propiogenic amino acid-restricted diet presented with an elevated blood mean corpuscular volume (MCV) of 93.1 fl, indicative of macrocytosis, and unusual hair texture with hypopigmentation. Plasma selenium concentration at this time was subnormal (45.9 micrograms/L), and calculated dietary selenium intake was 4.7 micrograms/day. After 4 months of selenium supplementation (50 micrograms/day) plasma selenium concentration normalized (97.7 micrograms/L) in conjunction with a reduced MCV (84.0 fl) and a dramatic improvement in hair growth, colour and length. Two additional periods off and on selenium supplementation, of varying time intervals, resulted in similar clinical changes. We conclude that these clinical changes were due to a deficient intake of dietary selenium.