Pregnancy-induced changes increase hazards associated with cyclosporine (CsA) treatment. Blood CsA trough levels (C0) were estimated in 15 pregnant renal allograft recipients treated with prednisolone + CsA + azathioprine using the TDx Abbott fluorescent polarization immunoassay. Despite therapeutical dose levels of CsA administered during pregnancy (3.52-3.67 and 3.59 mg/kg body weight in the first, second, and third trimesters, respectively), C0 significantly decreased (first trimester 130.8 +/- 36.9, second 92.0 +/- 32.7, and third 99.0 +/- 36.9 ng/ml). The mean increase of patient's body weight in mid-pregnancy was 3.0 +/- 2.19 kg and was associated with a significant (P < 0.05) fall in a hematocrit value (from 42 +/- 4.9% prior to pregnancy to 34 +/- 6% at the 20th week). We postulate that C0 concentration does not reflect the true exposure to CsA as no episodes of acute graft rejection were observed during pregnancy.