This study characterises the spectrum of biotinidase mutations in 21 patients (17 families) with profound biotinidase deficiency (BD) and 13 unrelated patients with partial BD using a denaturing gradient gel electrophoretic mutation screening and selective sequencing approach. In 29 from 30 unrelated families we found biallelic mutations including four common mutations, D444H (frequency 23.3%), G98:d7i3(20.0%), Q456H(20.0%), T532M (15.0%) and nine rare mutations (V62M, R157H, A171T+D444H, C423W, D543H, L279W, N172S, V109G, 12236G-A) with frequencies less than 5.0%. Only three profound BD patients with G98:d7i3/G98:d7i3 and Q456H/Q456H genotypes and residual biotinidase activities of 0.0%, and 0.9% of normal activity developed clinical symptoms before biotin supplementation at 8 weeks of age. All other patients remained asymptomatic within the first months of life or even longer without treatment. Two patients homozygous for the frameshift mutation G98:d7i3 had no measurable residual enzyme activity. Twelve patients with partial BD had the D444H mutation in at least one allele. We conclude that, based on mutation analysis and biochemical examinations of the enzyme, it is currently not clearly predictable whether an untreated patient will develop symptoms or not, although it seems that patients with activities lower than 1% are at a high risk for developing symptoms of the disease early in life.