Severe protein C deficiency in newborn infants.

Eleven infants initially seen in the neonatal period had levels of protein C suggestive of homozygous protein C deficiency but as an apparently acquired condition. Family studies failed to document parental carrier status, the clinical course was not typical of that reported with homozygous protein C deficiency, and protein C levels increased in all restudied infants, six of whom received heparin anticoagulation. No infant had evidence of vitamin K deficiency. Care is advised in the evaluation of infants with low levels of protein C. Parental blood studies, delayed testing, and serial assays can help to establish the correct diagnosis.