Paget's disease of bone (PDB) is a common disorder characterized by focal abnormalities of increased and disorganized bone turnover. Genetic factors are important in the pathogenesis of PDB, and in previous studies, we and others identified a locus for familial PDB by genome-wide search on 5q35-qter (PDB3). The gene encoding sequestosome 1 (SQSTM1/p62) maps to within the PDB3 critical region, and recent studies have identified a proline–leucine amino acid change at codon 392 of SQSTM1 (P392L) in French-Canadian patients with PDB. We conducted mutation screening of positional candidate genes in the PDB3 locus in patients with PDB, and also identified mutations in the gene encoding SQSTM1 as a common cause of familial and sporadic PDB. Three different mutations were found, all affecting the highly conserved ubiquitin-binding domain. The most common mutation was the P392L change in exon 8, which was found in 13 of 68 families (19.1%). Another mutation—a T insertion that introduces a stop codon at position 396 in exon 8—was found in four (5.8%) families. A third mutation affecting the splice donor site in intron 7 was found in one (1.5%) family. The P392L mutation was also found in 15 of 168 (8.9%) of patients with sporadic PDB and 0 of 160 of age- and sex-matched controls (P<0.0001). These studies confirm that mutations affecting the ubiquitin-binding domain of SQSTM1 are a common cause of familial and sporadic Paget's disease of bone.