<p>To evaluate the synergistic predictive capacity of MRI lesion classification and serum CA125 for symptom manifestation (dysmenorrhea/menorrhagia) and explore associated adenomyosis pathogenesis. Patients before focused ultrasound ablation surgery were categorized into MRI-based severity group (Group A) and MRI-based non-severity group (Group B). Binary logistic regression was employed to identify the factors associating dysmenorrhea and CA125 level in total cohort and subgroups via MRI-based classifications criteria. The receiver-operating characteristic (ROC) curve was applied to assess the utility of CA125 for dysmenorrhea. Patients in dysmenorrhea subtype exhibited higher CA125 levels compared with those in non-dysmenorrhea subtype in total cohort and Group A (<i>P</i> &lt; 0.05). Uterine volumes, lesion volumes, CA125 levels and symptom recurrence after FUAS in Group A were shown to be higher than those of Group B. In terms of those with dysmenorrhea, CA125 levels of Group A were shown to be higher when compared with those of Group B (<i>P</i> &lt; 0.05). In Group A, multivariate logistic regression showed that age and CA125 were related to dysmenorrhea in adenomyosis (<i>P</i> &lt; 0.05). CA125 level of 44.8 U/ml was demonstrated as the optimal cut-off point for dysmenorrhea by ROC curves in Group A and total cohort. In relative to patients whose CA125 ≤ 44.8 U/ml, those with CA125 levels﹥44.8 U/ml were more probably to suffer from dysmenorrhea. Furthermore, the multiple regression analysis demonstrated that CA125 level exhibited a positive correlation with the lesion volume and negatively related to age (<i>P</i> &lt; 0.05). Patients with CA125 &gt; 44.8 U/ml were prone to suffer from dysmenorrhea secondary to adenomyosis in MRI-based severity group. Besides, age and the lesion volume were associated with CA125 levels. These findings might offer clues for investigating the pathogenesis of dysmenorrhea and exploring the possible mechanisms with disease progression based on MRI and CA125.</p>