We encountered a patient with neuroblastoma showing remarkably high total LD activity (12,585 IU/l). His liver and heart function was normal. In the serum LD isoenzyme pattern, LD1 was increased, and LD2 extra band (LD2 ex) was observed on the negative pole side of LD2. However, LD2 ex was absent in erythrocytes of the patient, which demonstrates the exclusion of genetic factors. Neither the enzyme counter current method or immunofixation showed immune complexes, excluding anomalies. The LD isoenzyme in a metastatic lymphnode lesion before treatment showed increased LD1. The LD isoenzymes in the tissue, removed after 5 courses of chemotherapy, showed predominance of LD3 and absence of LD2 ex. Rapid decreases in the serum total LD activity and LD1 fraction and the disappearance of LD2 ex after chemotherapy suggest that these changes in the two LD fractions are associated with production of abnormal LD (LD2 ex) by the tumor.