31P NMR was used to study the erythrocytes of three patients who exhibited a familial multisystem disease characterized by fatty liver, diabetes and nonspherocytic hemolytic anemia of unknown etiology. 31P NMR measurements disclosed an abnormally high level of intracellular inorganic phosphate (Pi) and an abnormally low level of ATP in the erythrocytes 6 h after blood withdrawal from proband (I-1). This finding suggested that ATP was markedly decreased in the red cells of this proband, as compared with those of normal subjects. Time-dependent changes of 31P NMR spectra of the erythrocytes from the two daughters (II-1, II-2) of the proband demonstrated clearly an enhanced decomposition of ATP with a concomitant increment of Pi. Several ATP-consuming enzymes in erythrocytes, such as those in the Embden-Meyerhof system, pentose phosphate pathway enzymes, Na+, K(+)-ATPase and Ca2+, Mg2(+)-ATPase, were within normal limits of activity, but Mg2(+)-ATPase was drastically above the normal limit. The Mg2(+)-ATPase activity was 3 times higher in the red cell membranes of these patients than in those from normal subjects.