Background: The mitochondrial DNA mutation A3302G in the tRNA Leu(UUR) gene causes respiratory chain complex I deficiency. The main clinical feature appears to be a progressive mitochondrial myopathy with proximal muscle weakness. Objective: To report on clinical and molecular features in 4 novel patients with the A3302G mutation. Design: Case reports. Patients: Four patients (3 of whom are from the same family) with a myopathy caused by the A3302G mitochondrial DNA mutation. Main Outcome Measure: Identification of the A3302G mutation by DNA sequencing. Results: All 4 patients had an adult-onset progressive mitochondrial myopathy with proximal muscle weakness, resulting in exercise intolerance. In 2 unrelated patients, upper limb reflexes were absent with preservation of at least some lower limb reflexes. Other features including hearing loss, recurrent headaches, ptosis, progressive external ophthalmoplegia, and depression were present. Conclusion: While the dominant clinical features of the A3302G mutation were exercise intolerance and proximal muscle weakness, other features of mitochondrial encephalomyopathies, previously not described for this mutation, were present.