Three patients with congenital factor I deficiency associated with different clinical manifestations are described. Case 1 had one single episode of meningococcal disease, case 2 experienced four episodes of meningococcal disease and several other severe infections, whereas case 3, without known predisposition for infections, died from a subacute immune-complex mediated syndrome, resembling polyarteritis nodosa. Family studies in cases 1 and 2 revealed healthy individuals with factor I concentrations below the lower reference limit, indicating heterozygous carriers. The pedigree analyses were consistent with autosomal codominant inheritance. The estimated minimal frequency of the deficient gene was 0.002. Pedigree analysis was not performed in case 3 but the father and sister was found to be probable heterozygous carriers. Cases 2 and 3 were treated with infusions of freshly frozen plasma (FFP) (40 and 27 ml/kg bodyweight) during acute illness and the immunochemical complement profile was monitored. Following plasma infusion factor I was cleared from the circulation with a half-life of 29-45 h. The plasma infusions induced generation of C3d and C4d, increase in native factor B and C3 concentrations and disappearance of Ba split products. Native C3 and C4 increased to normal concentrations and remained normal till 16 days after the plasma infusions, whereas native factor B decreased to preinfusion levels 8 days after plasma infusion. It is concluded, that congenital factor I deficiency can present with different clinical manifestations and may be more prevalent than hitherto anticipated. Furthermore, infusion of blood products containing small amounts of functional factor I can partly normalize the complement profile, with a more prolonged effect on C3 and C4 than on factor B metabolism.