Between January 24, 1984, and December 31, 1988, 29 screening programs for biotinidase deficiency in newborns were established in 12 countries, and 4,396,834 newborns were screened. The worldwide incidence is based on screening programs in Australia, Austria, Canada, Italy, Japan, Mexico, New Zealand, Scotland, Spain, Switzerland, The United States, and West Germany. Biotinidase deficiency was detected in 72 newborns; 32 had profound biotinidase deficiency (less than 10% of mean normal activity level) and 40 had partial deficiency (10% to 30% of mean normal activity level). The combined incidence of profound and partial deficiency was 1 case per 61,067 live births (1:49 500 to 1:79 544; 95% confidence interval), the estimated frequency of the recessive allele was 0.0040, and the frequency of heterozygosity was estimated to be 1:123. Profound deficiency occurred in 1 per 137,401 live births (1:109,300 to 1:211,200), and partial deficiency in 1 per 109,921 live births (1:86,600 to 1:159,700). Most available parents of children with profound and partial deficiency had biotinidase activity levels intermediate between zero and mean normal activity levels. Six children with profound deficiency were symptomatic at, or soon after, the time of diagnosis; no infant with partial deficiency has become symptomatic, but little is known about the natural history of infants with partial deficiency. Most children whose biotinidase deficiency was detected by newborn screening were white, one was black, and one Hispanic; biotinidase deficiency has not been detected in Oriental children. Although 8 pilot programs have terminated, 21 will continue either indefinitely or until predetermined targets are reached, and 3 new programs were scheduled to begin in January 1989.(ABSTRACT TRUNCATED AT 250 WORDS)