Ten children with asymptomatic persistent cryptogenic increased serum levels of aspartate aminotransferase (AST) were screened for detection and monitoring of AST macroenzyme (macroAST). MacroAST was found in 4 patients; their serum AST levels were significantly higher than in those without biochemical evidence of macroAST (mean +/- SD: 515 +/- 433 and 78 +/- 16 IU/L, respectively; P = .0095). MacroAST was a persistent, benign phenomenon and was probably not congenital.