To investigate whether apparent diffusion coefficient (ADC) histogram parameters based on whole solid tumour volume could differentiate high-grade (HGSOC) from low-grade serous ovarian carcinoma (LGSOC) and to correlate those parameters with the Ki-67 proliferation index. One hundred and seven patients with HGSOCs and 19 patients with LGSOCs confirmed at surgery and histology who underwent conventional magnetic resonance imaging (MRI) and diffusion-weighted imaging (DWI) were analysed retrospectively. ADC histogram parameters (including the mean, standard deviation SD, 10th, 25th, 50th, 75th, and 90th percentiles, kurtosis, and skewness) were obtained using the whole solid tumour volume region of interest (ROI). The Mann–Whitney U test, Pearson's chi-square test, Fisher's exact test, kappa test, Spearman's correlation, and receiver operating characteristic (ROC) curves were used for statistical analyses. For ADC histogram parameters, the mean (p<0.001), SD (p=0.003), and all percentiles (10th, 25th, 50th, 75th, and 90th percentile; all p<0.001) were significantly lower in HGSOC than in LGSOC, and the area under the ROC curve (AUC) was 0.717–0.807. Skewness was significantly higher in HGSOC than in LGSOC (p<0.001, AUC = 0.773); however, kurtosis was not significantly different between HGSOC and LGSOC (p=0.140). The 25th and 75th percentiles, SD and 10th percentile, and 75th percentile showed the highest sensitivity of 91.6%, specificity of 79.0%, and accuracy of 88.1%, respectively. All histogram parameters (except for kurtosis) were poorly correlated with the Ki-67 index (|r| = 0.191–0.274, p<0.05). ADC histogram parameters based on whole solid tumour volume can be helpful for differentiating between HGSOC and LGSOC. •Most histogram metrics of ADC were significantly lower in HGSOC than in LGSOC.•The 75th showed the highest sensitivity and accuracy of 91.6% and 88.1%, respectively.•ADC histogram parameters showed a poor correlation with Ki-67 index.