A previously healthy 4-year-old girl exhibited butterfly like rash, heliotrope rash, Gottron’s papules, mild proximal muscle weakness, muscle grasping pain, and serum creatine kinase (CK) levels and erythrocyte sedimentation rate were elevated. This patient fulfilled the diagnostic criteria for dermatomyositis []. To fulfill the criteria, patients must meet more than 4 findings among 8 (1. proximal muscle weakness, 2. muscle grasping and spontaneous pain, 3. nondestructive arthritis or arthralgia, 4. elevated CK or aldolase level, 5. presence of systemic inflammatory signs (fever, elevated C-reactive protein, or elevated erythrocyte sedimentation rate), 6. myogenic changes on electromyogram, 7. anti Jo-1 antibody positive, 8. pathologic findings compatible with inflammatory myositis) with having shown the characteristic skin findings (heliotrope rash or Gottron’s papules). She was treated with high-dose oral prednisolone (PSL) followed by 2 courses of methylprednisolone pulse therapy (MPT). Subsequently, the skin rash and myositis gradually improved, and low-dose oral PSL was continued. Three months after MPT, she was emergently admitted to our hospital because of dyspnea, high fever, erythema, hepatosplenomegaly, cytopenia, liver dysfunction and coagulopathy. Hypoxemia, high Krebs von den Lungen-6 (KL-6) levels, and diagnostic imagings indicated progressive IP. Results of hematological examinations were as follows: leukocytes, 2.56 × 109/L; hemoglobin, 13.4 g/dL; platelet count, 119 × 109/L; D-dimer, 120.0 mg/L (reference range [rr]: 0–1 mg/L); alanine aminotransferase (ALT), 596 IU/L (rr: 5–43 IU/L); aspartate aminotransferase (AST), 1,154 U/L (rr: 12–34 IU/L); ferritin, 8,062 ng/mL (rr: 25–280 ng/mL); lactate dehydrogenase (LDH), 2,267 IU/L (rr: 115–217 IU/L); CK, 40 IU/L (rr: 41–123 IU/L); KL-6, 1,106 U/mL (rr: 0–499 U/dL). The urinary β2-microglobulin (U-β2MG) levels were elevated, 8.06 mg/L (rr: <0.29 mg/L). The tests for antinuclear antibody (1:80; speckle) and anti-melanoma differentiation-associated gene 5 (MDA5) antibody were positive. Anti-Jo-1 antibody and Epstein–Barr virus (EBV) DNA were negative. Bone marrow aspiration showed activated macrophages phagocytosing hematopoietic elements. She was diagnosed as having JDM-MAS, and progressive IP. Heparinization, dexamethasone palmitate (DP) and cyclosporine (CsA) therapy appreciably controlled the disease activity. Six days later, the flare-up of MAS such as decrease of platelet count and elevation of serum ferritin and U- β2MG levels required MPT for 3 consecutive days, and then successfully subsided. The following intravenous cyclophosphamide (IVCY) therapy led to a clinical and radiological remission of IP. We studied serum cytokine profiles in this patient during the acute (days 4, 6), subacute (day 12) or convalescent phase (day 50) of JDM-MAS with IP. Multi-target streaming protein quantitative technology (BD-Pharmingen Cytometric Bead Array; BD Biosciences, Franklin Lakes, NJ, USA) was used to detect the serum cytokine levels of interleukin (IL)-2,4,6,10, tumor necrosis factor-α (TNF- α), and interferon-γ (IFN- γ), following the manufacturer's instructions. The lower detection limits for IL-2, IL-4, IL-6, IL-10, TNF-α, and IFN-γ were 2.6, 2.6, 2.5, 2.8, 2.8, and 7.1 pg/mL, respectively. Serum levels of IL-18 were determined using enzyme-linked immunosorbent assay (ELISA) kits (Medical & Biological Laboratories, Co., Ltd., Nagoya, Aichi, Japan) according to the manufacturer's protocols. The lower detection limit was 12.5 pg/mL. The clinical courses and the results of cytokine expressions in our patient with JDM-MAS and IP are shown in Fig.,. There was hypercytokinemia before the treatment and during the acute phase of JDM-MAS with IP. IL-6 and IL-18 were at prominent high levels compared with the elevated levels of IL-10, TNF-α and IFN- γ. IL-6 levels promptly decreased after the initial therapy. IL-18 levels sustained to be high after the start of effective treatments during the acute and subacute phases of JDM-MAS. On the other hand, the IL-18 levels decreased after IVCY therapy, in concert with the declining KL-6 levels and the clinical and radiographical improvement of IP.