The deficiency of complement C5 is rare and frequently associated with severe and recurrent infections, especially caused by Neisseria spp. We observed the absence of component C5 in the serum of 3 siblings from a Brazilian family with history of consanguinity. The patients had suffered from recurrent episodes of meningitis and other less severe infections. Sera from these patients were unable to mediate hemolytic activity either by the classical or alternative pathways and presented extremely low levels of C5 protein (1.3, 0.9 and 1.0 μg/ml—normal range: 45–190 μg/ml). Hemolytic activity could be restored by the addition of purified C5 to deficient serum. Sequencing of sibling C5 cDNA revealed a homozygous 153 bp deletion that corresponds precisely to exon 30. The parents carried the same deletion but only in one allele. Sequencing of the corresponding region in the genomic DNA revealed a C to G substitution within intron 30 and, most significantly, the substitution of GAG4028 for GAA4028 at the 3′ end of exon 30 which is most likely responsible for skipping of exon 30. The resulting in-frame deletion in the C5 mRNA codes for a mutant C5 protein lacking residues 1289–1339. These residues map to the CUB and C5d domains of the C5 alpha chain. This deletion is expected to produce a non-functional and unstable C5 protein which is more susceptible to degradation.