Hereditary tyrosinemia type 1 (HT1) is an autosomal recessive disease caused by a deficiency of the enzyme involved in the last step of tyrosine degradation, fumarylacetoacetate hydrolase (FAH). Thus far, 34 mutations in the FAH gene have been reported in various HT1 patients. Site-directed mutagenesis of the FAH cDNA was used to investigate the effects of eight missense mutations found in HTI patients on the structure and activity of FAH. Mutated FAH proteins were expressed in Escherichia coli and in mammalian CV-1 cells. Mutations N16I, F62C, A134D, C193R, D233V, and W234G lead to enzymatically inactive FAH proteins. Two mutations (R341W, associated with the pseudo-deficiency phenotype, and Q279R) produced proteins with a level of activity comparable to the wild-type enzyme. The N16I, F62C, C193R, and W234G variants were enriched in an insoluble cellular fraction, suggesting that these amino acid substitutions interfere with the proper folding of the enzyme. Based on the tertiary structure of FAH, on circular dichroism data, and on solubility measurements, we propose that the studied missense mutations cause three types of structural effects on the enzyme: 1) gross structural perturbations, 2) limited conformational changes in the active site, and 3) conformational modifications with no significant effect on enzymatic activity.