Mutations in the inosine triphosphate pyrophosphohydrolase (ITPA) gene causing enzyme deficiency were shown to have pharmacogenetic implications in azathioprine-induced adverse drug reactions. The distribution of ITPA activity as well as the types and the frequencies of gene variants associated with a lower enzyme activity were determined in healthy volunteers from a Bulgarian population. The ITPA activity was measured in 185 erythrocyte samples by an established high-performance liquid chromatography procedure. All samples were genotyped for 94C > A, IVS2 + 21A > C, and IVS2 + 68T > C/G by real-time polymerase chain reaction with hybridization probes. The ITPA activity ranged from 7.5 to 587.8 micromoL IMP/(g Hb x h) with a median value of 162.9 micromoL IMP/(g Hb x h). The enzyme activity showed significant differences between females and males (P = 0.006) with 17% higher values in men than women. Mutant allele frequencies were 0.038 (94C > A) and 0.130 (IVS2 + 21A > C). Mutations at IVS2 + 68 were not identified. Using a cutoff at 75 micromoL IMP/(g Hb x h) phenotyping detected all heterozygous carriers of 94C > A, two compound heterozygotes, all IVS2 + 21A > C homozygotes and 12.5% of IVS2 + 21A > C heterozygous cases. A novel frameshift mutation 359_366dupTCAGCACC in exon 6 was found in a subject with reduced enzyme activity of 61.2 micromoL IMP/(g Hb x h). The interindividual variability in ITPA activity among the Bulgarian population resembles the distribution of enzyme activity in other whites, although the observed median activity was approximately 25% lower in the Bulgarians [163 vs 219 micromoL IMP/(g Hb x h)]. The most common mutant allele IVS2 + 21A > C showed a similar frequency like in other white populations, whereas the 94C > A mutation was less frequently observed compared with other whites. Heterozygosity for the novel gene variant 359_366dupTCAGCACC was associated with 30% enzyme activity of the wild-type median value. The role of this rare variant for the thiopurine intolerance is not explored. These data further extend the knowledge for ITPA heterogeneity in whites.