The UDP-glucuronosyltransferase UGT1 locus is composed of nine exon 1s, each flanked by a unique promoter region, and common exons (2, 3, 4, and the alternatively spliced exons 5a and 5b). Here, we characterized the genetic architecture of the UGT1 gene in a Caucasian sample. Overall, 98 variations in regulatory domains, exons and exon–intron boundaries were genotyped in 254 unrelated subjects, including 12 unreported UGT1 polymorphisms. We determined allele frequencies, computed pairwise linkage disequilibrium (LD), and inferred haplotypes; this thorough analysis yielding a limited number of common UGT1 haplotypes. Moreover, only 17 haplotype tagging single nucleotide polymorphisms (htSNPs) are required to capture most of the allelic diversity of the locus. Four haplotype blocks were inferred: Block 9/6 (UGT1A9, UGT1A7 and UGT1A6), Block 4 (UGT1A4), Block 3/1 (UGT1A3 and UGT1A1), and Block C (3′UTR). A high level of linkage exists between Blocks 9/6 and 3/1, while the 3′UTR SNPs are genetically isolated. The most common haplotype (16.5%) presents multiple deleterious alleles, mainly 1A1*28, 1A3*2, 1A6*2, and 1A7*4. More interestingly, we reveal the co-occurrences of multiple deleterious variations, some of which could be associated with interindividual differences in glucuronidation. Comparison with the HapMap data set demonstrated differences in haplotypic diversity between ethnic samples, but similarity between Caucasian cohorts, as observed previously. This report provides relevant data for further pharmacogenomic studies. Hum Mutat 0, 1–12, 2009. © 2009 Wiley-Liss, Inc.