Clinical and immunological findings of 5 patients with distinct defects in either humoral immunity or in the complement system are described. The syndromes presented comprise examples of type I dysimmunoglobulinaemia, X-linked agammaglobulinaemia (XLA), familial deficiency of complement factor C1q and a patient with a selective deficiency in the synthesis of antibodies against pneumococcal polysaccharides. The patients with a defect in humoral immunity all showed recurrent bacterial infections of the respiratory tract. The XLA-patient developed a dermatomyositis-like syndrome and ECHO-virus encephalitis. Prior to the development of a SLE-like syndrome the two siblings with C1q deficiency showed recurrent respiratory tract infections, most probably on basis of a defect in the clearance of immune complexes.