Type III galactosemia is a genetic disease caused by mutations in the gene encoding UDP‐galactose 4‐epimerase. A variety of different point mutations located throughout the gene can be responsible. The main, disease‐causing effects of these mutations appear to be a reduction in the catalytic rate constant (kcat) and an increase in the proteolytic sensitivity of the protein. Many of the mutations are distant from the active site of the enzyme and therefore must be assumed to affect the overall fold of the protein. Although the disease was previously classified into a severe, or generalized, form and an essentially benign, or peripheral, form this distinction has been blurred by recent work. Instead of two separate conditions it now appears that type III galactosemia is a continuum and that the symptoms will vary depending on the mutation(s) carried by the individual sufferer. This new way of looking at the disease has implications for the treatment and long term monitoring of patients. iubmb Life, 58: 83 ‐ 89, 2006