The efficacy of vitamin K prophylaxis (1 mg im or sc, or 1-2 mg orally both given as a single dose at birth) in the prevention of vitamin K deficiency bleeding in early infancy was estimated in Germany during a 15-month period between 1988 and 1989. Cases were identified by a survey of all paediatric hospitals and population denominators by a survey of all obstetric hospitals. Response rates were 85% and 68%, respectively. Thirteen cases of vitamin K deficiency bleeding in early infancy with confirmed prophylactic states were confirmed, seven of whom had intracranial haemorrhage. The estimated efficacy of single parenteral administration of vitamin K versus no prophylaxis was 96.7% (95% confidence interval: 74-99.6%) and for single oral administration versus no prophylaxis 80.4% (9.1-95.6%). Single parenteral vitamin K prophylaxis gave substantial protection against vitamin K deficiency bleeding in early infancy. Single oral prophylaxis appeared to be less effective, although the difference was not significant, as indicated by the wide overlap of the respective 95% confidence intervals.