Hereditary coproporphyria (HCP) is an autosomal dominant disorder, resulting from a partial deficiency of the enzyme coproporphyrinogen oxidase (CPO). This enzyme catalyzes the sixth step of the heme biosynthetic pathway, and mutations in the CPO gene have been coupled to HCP. The present study was undertaken to identify disease-producing mutations in the CPOgene in nine Swedish families with HCP. Exon 1 of the CPO gene of the nine probands was analyzed directly by sequencing, and exons 2-7 were screened by denaturating gradient gel electrophoresis, followed by sequencing of exons showing abnormal band pattern. Mutations were detected in five of the nine families. In two of these families, the novel mutations 623C>T (S208F, exon 2) and 982C>T (R328C, exon 5) were identified, respectively. In the affected members of the other three families, the previously reported mutations 991C>T (R331W, exon 5) and 1339C>T (R447C, exon 7) were shown to coexist on one allele. The present study contributes 2 novel mutations to the 34 that have been previously reported to cause HCP. In addition, this is the first report on patients carrying two HCP-coupled mutations on one allele.