Primary hyperoxaluria type 1 (PH1) is an autosomal recessive disorder of glyoxylate metabolism caused by deficiency of the hepatic peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT). The disease shows considerable phenotypic, enzymatic and genetic heterogeneity. To date, 7 polymorphisms and 11 point mutations have been described in the gene encoding AGT. We report on the prevalence of these polymorphisms and mutations in 79 patients with PH1 with the aim of assessing their diagnostic relevance. A strong association of the C154T, intron 1 insertion and C386T polymorphisms is confirmed and this linkage extends to include the type 1 variant of a polymorphic tandem repeat in intron 4. Only 64 of 158 (40%) PH1 alleles have one of the defined mutations, with the G630A mutation accounting for 39 of these and T853C for 14. Overall only 20 (25%) of the patients studied had the genetic basis of their disease fully explained: 7 were homozygous for the G630A mutation, 5 were homozygous for the T853C mutation, 1 was homozygous for the C819T mutation, and 7 had two different mutations identified and were presumed to be compound heterozygotes. Only the two more frequent G630A and T853C mutations are of general diagnostic relevance for mutation screening. It seems likely that there are a significant number of other mutations, perhaps family-specific, still to be described. There was no apparent difference in the types of mutations in patients presenting in the first year of life (36%), suggesting that other factors, such as periods of dehydration or urinary tract infections, might contribute more to the clinical manifestation than genotype.