Two trials of external quality assessment for inborn error of metabolism was performed in 1999. In Trial 1, 6 specimens for neonatal screening test were distributed to 56 laboratories with a response rate of 78.6%. Three specimens were distributed to 64 laboratories with 50 returns (78.1%) in Trial II. All the control materials were sent as a filter paper form. Each laboratory replied the test result as the screening items they were doing as a routine test at the reception of the specimen among PKU screening, neonatal TSH, neonatal T4 (total, free), galactosemia screen, 17-OH progesterone, maple syrup urine disease (MSUD) screen, homocystinuria screen and histidinemia screen. The median, range, mean, SD, and CV were analyzed.