Sick low birth weight infants (LBWI) are prone to develop rapid onset of essential fatty acid (EFA) deficiency. EFAs serve as precursors for prostaglandins (PGs). We measured the excretion of the major urinary metabolite of prostaglandins E1 and E2, 7alpha-hydroxy-5,11-diketotetranorprostane-1,16-dionic acid (PGE-M), in three EFA-deficient and in nine thriving neonates. There was no significant difference in PGE-M excretion between the sexes among thriving infants nor did PGE-M excretion appear to be affected by postconceptual age. However, a significant difference between the PGE-M excretion in the group of infants with EFA deficiency before and after treatment is apparent (P less than 0.05). Significant differences in PGE-M excretion were also found between the control group and the EFA-deficient infants. The severity of the EFA deficiency correlates directly with the degree of PGs excretion. The biochemical evidences of EFA deficiency and the decreased levels of PGE-M excretion are rapidly corrected when patients resume a diet containing EFA.