A 34-year-old woman with congenital factor XIII (FXIII) deficiency and multiple connective tissue tumours is reported. The subunit a of FXIII was totally absent in her plasma, platelets and histiocytes of breast fibroadenomas and considerably reduced in the monocytes (below 5%). The plasmatic level of subunit b was also reduced (25%). She had a bleeding tendency and habitual abortions. Fresh frozen plasma therapy permitted a successful pregnancy.