Dolichyl monophosphate (Dol-P) is involved in the attachment of carbohydrate chains to proteins in the formation of N-linked glycoprotein. We found that this compound induces apoptosis in human leukemia U937 cells. During this apoptotic execution, the increase of plasma membrane fluidity (5–20 min), reduction in mitochondrial transmembrane potential (Δψm) and translocation of apoptosis-inducing factor (1–3 hr), caspase-3-like protease activation (2–4 hr), chromatin condensation and DNA ladder formation (3–4 hr) were observed successively. In this study, we examined mitochondrial morphological changes by electron microscopy and Δψm by JC-1 from immediately after treatment of Dol-P. After 5 min of treatment, we observed clearly that mitochondrial cristae began to be disrupted ultrastructurally and almost all the cristae were disintegrated after 1 hr of treatment. The Δψm of Dol-P treated cells was reduced to 34% as compared with that of control cells immediately after treatment and was quartered within 1 hr. The reduction in Δψm was not inhibited by cyclosporin A, N-acetyl-L-cysteine and vitamin E. These results indicate that mitochondrial disruption is one of the first triggering events of Dol-P-induced apoptosis.