Propionic acidaemia (PCCD) or deficiency of propionyl-CoA carboxylase (PCC) is one of the most common organic acidaemias. Recent studies have suggested that this disease can cause somatic or cognitive deterioration even in patients without ketosis or metabolic acidosis, or in cases with unusually late onset. This suggests that for this disease a sensitive yet practical screening procedure is required to achieve early treatment. We conducted a pilot study of gas chromatographic-mass spectrometric screening of 12,000 newborns for PCCD using eluates from dried filter-paper urine collected at 4-7 days of age. Methylcitrate (MC) was targeted for PCCD. For bulk screening, 2-hydroxyundecanoate was used as internal standard; for quantification, stable-isotope-labelled MC was used. Urease pretreatment without fractionation allowed satisfactory recovery and reproducibility of the highly polar MC. We detected an asymptomatic male infant with distinctly elevated MC: the creatinine-corrected level relative to 2-hydroxyundecanoate was 4.8 SD above the normal mean. The MC concentration calculated using the stable-isotope-labelled internal standard was 70.6 mmol/mol creatinine 14.7 SD above the normal mean of 3.70. Parallel analysis of the dried blood spot at 4 days of age by tandem MS showed only borderline elevation of propionylcarnitine. The activity of PCC in lymphocytes was 7% of control. Gene analysis revealed that a single missense mutation, TAT to TGT, resulting in Y435C in the beta chain was present in a homozygous form. Dietary treatment including carnitine supplementation decreased this infant's MC level and to date (at 13 months of age), he shows no neurological or somatic abnormalities.