During a five-year-period (1985-1989) 420,000 newborns in Lower Saxony, FRG, were screened for biotinidase deficiency using biotinyl-para-amino-benzoic-acid as substrate. Three newborns with profound biotinidase deficiency (activity 1.1%, 2.1%, 2.3% of mean normal activity level) were detected. Nine newborns had partial biotinidase deficiency (activity 17-26% of mean normal activity level), thus giving an incidence of 1:140,000 with profound, and 1:46,667 with partial biotinidase deficiency, respectively. The infants with profound biotinidase deficiency are treated with biotin (2 x 5 mg/day) from the 3rd, 6th and 8th week of life and have developed normally so far. The children with partial biotinidase deficiency are not treated but followed up closely. The necessity of newborn screening for biotinidase deficiency is stressed.