The causes and effects of transient neonatal ketosis, discovered during a pilot study of screening for abnormalities in neonatal metabolism using gas chromatography-mass spectrometry, were investigated. Of the 21,342 neonates that were screened, 47 had significant ketosis. The organic acid profile accompanying ketosis in the urine of neonates followed the pattern of ketotic dicarboxylic aciduria in approximately half of the cases. Ketosis was more often found in neonates nourished by breast feeding (33 out of 47). Over half of the neonates showing ketosis (28 out of 47) were asymptomatic. When normal neonates and neonates testing positive for ketosis were compared, no statistically significant correlations were found with regard to birth mass, gestational period, or gender. However, neonates with ketosis tended to have low mass gain rates in the 5 days from birth and a statistically significant difference was found in this regard in comparison to normal neonates (P<0.0001). From the above results, development of ketosis in neonates was found to be possible even in normal subjects. Most ketosis in neonates was also found to depend largely on nourishment after birth. Existence of an asymptomatic ketosis category was also suggested.