Nine cases of childhood malignant histiocytosis (MH) showed an abnormally high serum phenylalanine (Phe)/tyrosine (Tyr) ratio (3.47 +/- 1.32) coincident with hyperferritinemia (50,800 +/- 33,600 ng/ml). Lactate dehydrogenase activity was also increased in these patients. These values were compared with data on sera from two groups of patients, acute leukemia cases (n = 14) and measles cases (n = 13), and with control values from normal healthy children (n = 38). The Phe/Tyr ratio was 1.57 +/- 0.54 for the acute leukemia (p less than 0.01) and 2.58 +/- 1.46 for the measles cases (NS), serum ferritin was 245 +/- 124 ng/ml for acute leukemia (p less than 0.01) and 167 +/- 117 ng/ml for measles (p less than 0.01). Accordingly, the concurrence of both abnormalities is considered to be characteristic for MH. It was also found that both serum Phe/Tyr ratio and ferritin levels reflect the disease activity, indicating that these two factors are useful prognostic indicators in the treatment of patients with MH.