BACKGROUND Adenomyosis is a cause of chronic pelvic pain in women of reproductive age. The aim of this study was to investigate the effects of subcutaneous etonogestrel implantation on adenomyosis. MATERIAL AND METHODS A clinical observational study included 17 women with adenomyosis who were treated with subcutaneous etonogestrel implants and followed-up for 12 months. Imaging and clinical observations were undertaken in the 17 patients at baseline (time 0), and at 3 months, 6 months, and 12 months following subcutaneous etonogestrel implantation. The following imaging and clinical findings were compared between baseline (time 0) and 12-month follow-up: menstrual bleeding pattern, dysmenorrhea, visual analog scale (VAS) pain score, uterine volume, serum cancer antigen 125 (CA125) levels, hemoglobin, follicle-stimulating hormone (FSH) levels, luteinizing hormone levels, serum estradiol levels, and any treatment side effects. RESULTS All 17 patients treated with etonogestrel implants completed the 12-month follow-up, at which time, the mean hemoglobin level (127.08±2.56 g/L) was significantly higher compared with that at baseline (94.54±5.47 g/L; P<0.01); uterine volume, serum CA125, and VAS score for dysmenorrhea at 12 months (118.03±12.83 cm³, 34.58±9.66 U/mL, and 1.45±0.35, respectively) were significantly lower when compared with baseline (198.53±39.47 cm³, 100.41±49.89 U/mL, and 7.62±0.74, respectively) (P<0.01, for all). However, changes in bleeding pattern and amenorrhoea occurred after treatment in some women. CONCLUSIONS Subcutaneous etonogestrel was effective in reducing some symptoms and signs of adenomyosis, including dysmenorrhea, anemia, serum CA125, and uterine volume.