An 8-year-old girl complained for 4 months of right arm pain, weakness in both legs, difficulty in arising from a seated or squatting position, and 1 month of pain in her hips, ankles, and knees. On physical examination, she had weak neck flexors, weak proximal and abdominal muscles, and an assisted Gower maneuver; both knees and ankles were painful. Erythematous macules on her elbows, knees, and medial ankles were present without heliotrope rash or dilated eyelid capillaries. She had nail-fold erythema and decreased numbers of nail-fold capillary end-row loops (ERLs) (5.42 ERLs per mm [normal: ≥6.8 ERLs per mm]) without digital ulcers or tight skin. Laboratory testing revealed slightly elevated creatine phosphokinase (440 IU/L [normal: ≤199 IU/L]) and aldolase (11.7 U/L [normal: ≤8.6 U/L]) levels. Her eosinophilia (7.2%) was not characteristic of juvenile dermatomyositis. Rheumatologic evaluation included a positive antinuclear antibody test result (1:5120 titer), speckled pattern (normal: <80 titer), myositis-associated and -specific antibodies that showed indeterminate Mi-2, with the others negative, including p155/140, elevated immunoglobulin G (IgG) (1440 mg/dL [normal range: 608-1229]) and IgE (409 kU/L [normal: <160 kU/L]) levels, and normal levels of IgM and IgA. She had an increased neopterin level (20 nm/L [normal: <10 nm/L]) and decreased absolute count of CD3-CD56/16(+) natural killer cells (89 [lower normal limit: 138]). MRI of her thigh muscles revealed serpiginous increased T-2 signals consistent with inflammation and a complex round mass in the left pelvis. A muscle biopsy did not indicate juvenile dermatomyositis. Pelvic ultrasound confirmed a solid mass of the left ovary consistent with a mature teratoma. After surgical removal of the teratoma, the myositis, synovitis, and cutaneous findings resolved over 4 months without further therapy.