Familial Mediterranean fever (FMF) is an inherited inflammatory disease that is frequently complicated by reactive systemic (AA) amyloidosis. It is principally recognized in certain Mediterranean populations, and the diagnosis depends on clinical features. Four mutations strongly linked to FMF have lately been identified in a gene encoding a novel protein that has been named pyrin or marenostrin. We studied 27 consecutive patients of varied ethnic origin, including an English man, who had classical, probable or possible FMF. Pyrin/marenostrin genotypes were determined, and AA amyloidosis was sought using serum amyloid P component scintigraphy. Among the 23 patients with classical or probable FMF, 17 were homozygotes or compound heterozygotes for pyrin/marenostrin mutations, and in five, only single allele mutations were identified. Two new mutations, T6811 and delta M694, were discovered in addition to the four described previously. No mutations were identified in three of the four patients with possible FMF. Nine patients had AA amyloidosis, but this association was not restricted to any particular genotype. Most patients with FMF have mutations in both pyrin/marenostrin alleles, and genotyping at this locus is a valuable diagnostic test. Unidentified second mutations are likely to occur in FMF patients who have apparently solitary mutations, and therefore genotype results must be interpreted in conjunction with the clinical picture.