OBJECTIVE: To determine the association of decreased lung function in children with idiopathic inflammatory myopathies (IIMs) with specific clinical parameters. METHODS: This study of 38 children ages 6-23 years diagnosed with definite/probable IIM evaluated the association of myositis-specific/-associated antibodies (MSAs/MAAs), duration of untreated disease at diagnosis, Disease Activity Score for muscle (DAS-M), muscle-derived enzymes (aldolase, lactate dehydrogenase [LDH], aspartate transaminase, and creatine phosphokinase [CPK]), neopterin and von Willebrand factor antigen, and the Childhood Myositis Assessment Scale (CMAS) scores with data from pulmonary function testing (PFT). RESULTS: Impaired PFTs were defined as total lung capacity (TLC) or diffusing capacity for carbon monoxide (DLCO) of <80% predicted. The PFTs documented that 37% of the children (14 of 38) had either decreased TLC or decreased DLCO; 5% (2 of 38) had both. Children with decreased TLC alone (7 [18%] of 38) were older both at the time of PFT and diagnosis, had anti-Jo-1 and anti-Scl-70 antibody, and had elevated levels of CPK and neopterin. Children with decreased DLCO alone (5 [13%] of 38) had a shorter duration of untreated disease at diagnosis, had higher DAS-M and total DAS, were positive for anti-Ro and anti-PL-12, had increased LDH, and had elevated levels of neopterin and aldolase, with low CMAS scores for items 1, 3, 10, 11, and 14. CONCLUSION: Assessment of PFTs in children with IIMs should be considered, since more than one-third of patients were found to be impaired. The presence of MSAs/MAAs, an elevated serum neopterin level (mean +/- SD 12.4 +/- 9.6 nmoles/liter, normal value <10.5), older age at diagnosis, and shorter duration of untreated disease at diagnosis suggest the presence of potential lung pathology.