In up to 50% of recurrent pregnancy losses (RPL), the underlying pathophysiological mechanisms remain undetermined. Immunological factors may be involved. The objective of this Study was to compare serum concentrations of autoantibodies to carbonic anhydrase (CA) I and II in women with and without RPL. Serum anti-CA-I and anti-CA-II 1 concentrations in 29 women with RPL and 39 women without RPL on menstrual cycle day 3 were assessed and compared for this prospective age-matched controlled Study. The mean serum anti-CA-I (0.287 +/- 0.177 versus 0.184 +/- 0.0931 P = 0.003) and anti-CA-II (0.496 +/- 0.240 versus 0.344 +/- 0.108, P = 0.001) antibody concentrations were significantly higher in women with RPL, compared with controls. For serum anti-CA-I, an absorbance higher than 0.463 was taken as positive (mean + 3 SD of controls) and was detected in four of 29 patients with RPL (13.79%, 0.287 +/- 0.177). For serum anti-CA-II, an absorbance higher than 0.668 was taken as positive (mean + 3SD of controls) and was detected in seven of 29 patients with RPL (24.14%, 0.496 +/- 0.240). All patients with positive anti-CA-I antibody also had positive anti-CA-II antibody. Antibodies specifically reactive to CA-I and CA-II were found to be present at I higher frequency in the serum of subjects with RPL.