We have identified two classes of fusion events in mammalian cells: complete fusion and transient fusion. In transient fusion, two mitochondria exchanged soluble intermembrane-space and matrix proteins without equilibration of the integral membrane proteins and reseparated preserving the original morphology. Although Opa1, the inner mitochondrial membrane fusion protein is required for both complete and transient fusions, less Opa1 is sufficient to support transient fusion. To understand the specific role of transient fusion in mitochondrial maintenance, we searched for human conditions that display Opa1 loss. Human mutations in Opa1 are associated with Opa1 depletion and cause autosomal dominant optic atrophy (ADOA). Some Opa1 mutations result in mitochondrial fragmentation but the patients’ mitochondrial metabolism is well preserved and the clinical symptoms are mild. We reasoned that transient fusion might be retained and support mitochondrial metabolism in these patients. Here, we studied skin fibroblasts derived from a patient who carries the mutation c.984G>A in the GTPase domain of Opa1 and has only weak visual and metabolic impairments, and fibroblasts of two unaffected individuals. In cells bearing Opa1c.984G>A, the two major Opa1 bands were reduced. The majority of the Opa1c.984G>A fibroblasts showed fragmented mitochondria (partial 45%, complete 17%) compared with elongated mitochondria in the controls. In c.984G>A cells with partially fragmented mitochondria, most fusion events were transient, whereas complete fusion dominated in the control cells. Strikingly, the deltapsi, ATP level and mtDNA content were maintained in Opa1c.984G>A cells. Thus, transient fusions are spared and seem to be sufficient to support mitochondrial metabolism in Opa1c.984G>A fibroblasts, exposing a mechanism that may contribute to the lack of severe clinical symptoms in ADOA associated with some Opa1 mutations.