Mitochondrial DNA deletions have been described in the Kearns-Sayre syndrome (KSS) and the Pearson's marrow-pancreas syndrome. In some cases, the same 4,977-bp deletion has been identified in these two very different diseases. Therefore, it is not currently possible to predict the clinical phenotype from the size or location of the deletion. Instead, differential tissue distribution of the deletion has been implicated as one possible determinant of phenotype. In particular, in KSS the deletions have not been detected by Southern blotting in the blood, whereas in Pearson's syndrome they are easily detectable. We describe here an 11-y-old boy with clinically characteristic KSS and a 7.4-kb mitochondrial DNA deletion between nucleotides 7,194 and 14,595. Southern blotting reveals that 75% of the mitochondrial DNA molecules from his peripheral blood have this deletion. This case blurs further the molecular distinction between the KSS and Pearson's marrow-pancreas syndrome, questioning whether tissue distribution is a sufficient explanation for the very different phenotypes of these disorders.