The autosomal recessive disease primary hyperoxaluria type 1 (PH1) is caused by a functional deficiency of the liver-specific peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT). An analysis of liver samples from 59 PH1 patients showed considerable heterogeneity at the enzymic level. Approximately two thirds of patients had zero AGT catalytic activity, whereas the remaining one third had activities that ranged from 3% to 48% of the mean normal level. Two thirds of patients with zero AGT activity also had zero immunoreactive AGT protein, while the other one third, together with all the patients with detectable AGT catalytic activity, had levels of immunoreactive AGT protein that varied from normal to only a few percent of normal. All patients with AGT catalytic activity had their enzyme in the wrong intracellular compartment (ie, mitochondria). On the other hand, in all but one of the patients with immunoreactive AGT protein, but zero catalytic activity, the inactive AGT was correctly located within the peroxisomes. This enzymic heterogeneity was matched by considerable heterogeneity at the clinical level (eg, age at onset, rate of progression, age at renal failure, etc). No simple relationship was found between the level of hepatic AGT and the severity of the disease. It is suggested that a lack of AGT might be responsible for a broader pathological phenotype than classically associated with PH1. The possibility is advanced that some patients with idiopathic oxalate stone disease might owe their predisposition to stone formation to a functional deficiency of AGT.