Proteins containing gamma-carboxyglutamic acid (Gla) are present in subcutaneous calcifications of adults with dermatomyositis or scleroderma. Sixteen children with juvenile dermatomyositis, including 7 with subcutaneous calcifications, were studied to determine if abnormal synthesis or turnover of Gla-containing proteins occurred. All study children had increased excretion of the amino acid that was greater than that of age- and sex-matched controls. Patients who had juvenile dermatomyositis with calcifications had a 3-fold increase in Gla excretion, and those without calcinosis had a 2-fold increase. Five other children with various connective tissue disorders and subcutaneous calcification had 2-fold increased Gla excretion. Decreased excretion of this amino acid was associated with salicylate therapy (80 mg/kg/24 hours). The data suggest an abnormal turnover of Gla-containing proteins in juvenile dermatomyositis. Metabolism of these proteins may be involved in the pathophysiology of soft-tissue calcification in children.