BACKGROUND AND OBJECTIVES: Mitochondrial diseases are caused by defects in oxidative phosphorylation, with primary mitochondrial myopathies (PMM) being a subset where muscle involvement is predominant. PMM presents symptoms ranging from exercise intolerance to progressive muscle weakness, often involving ocular muscles, leading to ptosis and progressive external ophthalmoplegia (PEO). PMM can be due to variants in mitochondrial or nuclear DNA. Growth differentiation factor 15 (GDF15) has been identified as an accurate biomarker for mitochondrial dysfunction. This study evaluates the utility of GDF15 as a biomarker for monitoring PMM. METHODS: This observational study involved 50 adult PMM patients. Clinical data were collected alongside functional motor outcomes measured by the Motor Research Council scale, 6-min walk test, North Star Ambulatory Assessment, and 100-m run test (100MRT). Biomarkers including serum lactate, creatine kinase (CK), creatinine, and plasma GDF15 were assessed. RESULTS: Patients exhibited diverse phenotypes, including exercise intolerance (8 %), progressive myopathy (22 %), isolated PEO (24 %), and PEO plus (42 %). Significant correlations were found among motor function tests, with 100MRT being particularly sensitive. Biomarker analysis showed elevated lactate in 32 %, elevated CK in 54 %, reduced creatinine in 76 %, and elevated GDF15 in 86 % of cases. GDF15 levels correlated with motor performance, lactate levels, and mtDNA mutation load in muscle. Creatinine levels were strongly linked to disease severity. DISCUSSION: This study underscores the heterogeneity of PMM and the importance of reliable biomarkers. GDF15 was consistently elevated across all PMM phenotypes and genotypes, correlating well with disease severity. Reduced creatinine also emerged as a potential prognostic marker.