The levels of diadenosine tetraphosphate hydrolase, a nucleotide pyrophosphatase (EC 3.6.1.9), were measured in human sera with a bioluminescence method. 40 sera of healthy donors and 207 samples obtained from inpatients of a medical clinic were analysed. About two thirds of the patients showed increased hydrolase levels, as compared to the normal donors. Elevated levels were not specific for a certain disease, and they were found in quite different disorders. There was no indication for a dependency of enzyme activity on age or sex of the patients. The comparison of diadenosine tetraphosphate hydrolase with seven other laboratory parameters revealed a highly significant correlation with gamma-glutamyl transferase, suggesting that the liver may be a source of elevated serum activities. There was evidence that the gastrointestinal tract and the pancreas may also be regarded as possible organ sources of the hydrolase.