We investigated feasibility of using the formalin-fixed and paraffin-embedded tissue to study mitochondrial mutations in the case that fresh or frozen tissue, or blood samples are not available. Four paraffin blocks of muscle biopsies in Korean MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) patients were chosen. Total DNA was extracted from these blocks for PCR/RFLP analysis, and sequencing was performed to study the most common mutation, A to G transition at nucleotide position 3243 underlying MELAS in the mitochondrial tRN $A^{Leu(UUR)}$ gene. We could identify the A to G mutation at nt.3243 in three MELAS patients. Our results show that the mitochondrial genome of our paraffin blocks is presumably in good condition. Our results are in accordance with the previous findings by other investigators that PCR allows molecular genetic analysis of paraffin-embedded tissues stored in most histopathology laboratories.s.