Five patients with 3-hydroxydicarboxylic aciduria have been investigated. Two of them had elder siblings who had died unexpectedly in early infancy. Stored filter paper blood samples obtained from the patients and their siblings for neonatal screening were retrieved. Elevated levels of 3-hydroxy fatty acids were observed in the samples from three of the five patients with 3-hydroxydicarboxylic aciduria and in the samples from both siblings.