The tyrosinemias are a complex and heterogeneous group of disorders in tyrosine catabolism that embrace a wide spectrum of clinical conditions, ranging from the benign neonatal variety to the severe hepatorenal form. Readily available diagnostic tests are too insensitive to distinguish between these variants, and more definitive but technically difficult tests can be performed rapidly in only a few centres. Effective management may therefore be compromised, due to the inability of obtaining a working diagnosis quickly. This report describes difficulties encountered with conventional testing in three patients. Analysis of whole blood delta-aminolevulinic acid dehydratase activity and determination of urinary inhibition activity against the enzyme were found to be rapid and reliable screening tests for hepatorenal or type I hereditary tyrosinemia. These procedures are recommended in the initial evaluation of undifferentiated tyrosinemic states.