The objective of this study was to establish whether or not patients with unexplained recurrent abortion have an increased incidence of haemostatic or metabolic abnormalities. Fifty-two patients with a history of unexplained habitual abortion (two or more spontaneous abortions before 16 weeks' gestation) were tested for protein S, protein C and antithrombin (AT) III deficiency, activated protein C (aPC) resistance, hyperhomocysteinaemia and anticardiolipin antibodies (ACA). The control group consisted of 67 healthy women with a history of only uncomplicated pregnancies. Blood samples were taken for measuring protein S, protein C, AT III, ACA and activated protein C resistance and a methionine loading test was performed. Of the 46 patients tested for protein S deficiency, 8 (17.4%) were positive. Of the 43 patients tested, two (4.7%) were protein C deficient and none was AT III deficient. Of the 42 patients tested for ACA, eight (19.1%) had detectable antibodies. Of the 44 patients tested for aPC resistance, two (4.6%) were positive. Finally, 35 patients were tested for hyperhomocysteinaemia and six (17.1%) were positive. It was concluded that parous women with a history of unexplained recurrent abortion have an increased incidence of hyperhomocysteinaemia and a trend of increased incidence of ACA can be found.