Hereditary tyrosinemia type I (HTT-I) is an inherited metabolic disorder with severe liver disease and a high risk for hepatic malignancy. Patients with HTT-I are monitored with repeated analyses of serum amino acids, urine succinylacetone, and serum α-fetoprotein (AFP). Oncofetal markers CA 125 and CA 19–9 are elevated in serum of patients with various gastrointestinal diseases and malignancy. To study the biology of oncofetal antigens in tyrosinemia and to assess the possible usefulness of these markers in HTT-I, we studied serum concentrations of CA 125 (n = 160) and CA 19–9 (n = 188), together with AFP (n = 337), in serial samples from 10 patients. At early stages of the disease, most children with an acute type of disease had a remarkably elevated serum CA 125 concentration (153–1560 IU/L) that normalized gradually after the institution of therapy. Serum CA 125 levels may thus reflect acute metabolic imbalance in fulminant HTT-I. The patients with a chronic type of disease showed CA 125 levels within the normal range at diagnosis that slowly increased as the liver condition worsened. These concentrations, however, never reached values seen in acute HTT-I. Serum concentration CA 19–9 in HTT-I was mostly normal. Serum AFP levels fluctuated in all patients and positively correlated with tests for metabolic state and biliary function. A distinct increase in the serum AFP level was recorded in association with the detection of massive hepatocellular carcinoma and also preceded metabolic imbalance leading to porphyria crises. Fluctuations in serum AFP concentration, however, limit its usefulness in early detection of developing hepatic malignancies or porphyria crises in HTT-I. Serum CA 125 or CA 19–9 are also unlikely to offer diagnostic tools for detecting developing hepatocellular carcinoma in HTT-I.