Erythropoietic protoporphyria (EPP) is an inherited disorder, caused by a partial deficiency of ferrochelatase (FECH), the last enzyme of the heme biosynthetic pathway. The deficiency results in accumulation of protoporphyrin, primarily in erythroid cells, and the major clinical feature is cutaneous photosensitivity. In addition, some patients may develop liver complications. Several EPP-coupled mutations have been identified in the FECH gene, and the less than 50% of FECH activity seen in patients with overt EPP was recently shown to be due to the in trans inheritance of one deleterious mutation and a IVS3-48T>C transition in intron 3 of the FECH gene. This IVS3-48T>C transition modulates the use of a constitutive aberrant splice site, which results in a decreased FECH mRNA level in the carrier. In the present study, the inheritance of four novel (364C>T, 393delC, 532G>A, and 1088-89insGG) and two previously reported (343C>T and 1001C>T) FECH mutations, and the splice site modulator IVS3-48C was investigated in nine Swedish families with EPP. The methods used for the FECH gene analysis included denaturating gradient gel electrophoresis, sequencing analysis, and restriction enzyme cleavage. Haplotype analysis, based on the polymorphic loci 287(G/A), IVS3-48(T/C), and 921(G/A), revealed that all individuals carrying a mutated allele and IVS3-48C in trans to each other were affected by overt EPP. Mild clinical and biochemical EPP signs may, however, be present in individuals carrying a T at position IVS3-48 in trans to a mutated allele, because this was the case in one of the individuals investigated in the present study.