In a period of 40 months (1st March 1999 to 30th June 2002) 610,000 blood samples were analysed in one screening centre for citrulline as a pilot study for neonatal screening using tandem mass spectrometry. Persistent hypercitrullinaemia (Cit >1.5 mg/dl or 85.5 micro mol/l, not corrected for recovery) was identified in 15 newborns. Four children were diagnosed with classical neonatal onset citrullinaemia and eight with persisting asymptomatic hypercitrullinaemia. In two asymptomatic newborns and in one symptomatic preterm patient, argininosuccinate lyase deficiency was identified as the cause of moderately elevated levels of citrulline (cases not described in this paper). Citrulline concentrations were only temporarily mildly elevated in two newborns and in these the results of the original neonatal screening were therefore regarded as false-positive; we did not find any other false-positives. The screening result allowed the introduction of immediate specific treatment in two cases of citrullinaemia and may have prevented metabolic decompensation in those with presumed mild citrullinaemia. In one child who developed severe hyperammonaemia on the 2nd day of life, sequelae could not be avoided. CONCLUSION: neonatal screening for citrullinaemia is more complex than expected and, with the actual logistics, results may be obtained too late in severe forms.