We developed mitochondrial (MT) scoring system based on MT ultrastructural findings in association with response to chemotherapy in ovarian cancer (OC). Ultrathin sections of MT prepared from 28 OC patients before chemotherapy were examined by electron microscopy. Platinum-sensitive ovarian carcinoma cell line 2008 and its resistant variant C13 were used as control cells. Seven independent MT features including, diameter, pattern of cresta structure, electron density, distribution-density and -pattern, ratio of minimal/maximal diameter and MT architecture were examined and were assigned a score between 0 and 2. Twenty-eight cases were primary advanced OC, including 4 recurrent cases. Nine cases were chemosensitive while 19 were resistant. Univariate and multivariate analysis in each factor showed good correlation to chemosensitivity for 2 factors of electron density, distribution pattern. Total score of these 2 factors in 9 sensitive cells was 1.44+/-0.41 (M +/- SE) and was 3.58+/-0.18 in 19 resistant cells (P<0.001). Receptor operative characteristics (ROC) analysis revealed that total 'cut-off' score was 3 point (P<0.05; AUC=0.84). In conclusion, this MT scoring system was excellently correlated to response regardless of histopathologic findings and this strongly suggests that the system is deemed to be of great value as biomarker for the chemosensitivity in OC.