Progressive hepatocellular dysfunction in a neonate, resulting in elevated serum alpha-fetoprotein together with raised blood levels of tyrosine and methionine, a generalized amino aciduria and the absence of urinary delta-aminolevulinic acid and succinylacetone, suggests a diagnosis of tyrosinemia type Ib. Classical tyrosinemia type I arises from a deficiency of fumarylacetoacetate hydrolase while the variant tyrosinemia type Ib results from a deficiency of maleylacetoacetate isomerase.