The objectives of this study were to determine whether antiphosholipid antibodies are associated with in vitro fertilization (IVF), and assess the potential biological effects of these antibodies. Sera from seventy infertile women (18 before IVF, 13 submitted to one IVF cycle and 39 after three cycles) and 28 healthy controls were collected. Anticardiolipin (anti-CL) and antiphosphatidylserine (anti-PS) antibodies, paraoxonase (PON) and Total Anti-oxidant Capacity of plasma (TAC) were measured. Anti-CL and anti-PS titres were significantly increased in treated patients compared with patients before treatment or controls (P < 0.001). There were no differences regarding anti-CL and anti-PS titres between controls and untreated patients nor when different types of infertility were considered. PON activity and TAC were significantly reduced in treated patients when compared to untreated and controls (P < 0.001; P < 0.002). PON correlated inversely with anti-CL and anti-PS IgG (r = - 0.734; P < 0.001) and directly with TAC (r = 0.720, P < 0.001). In conclusion PON activity is decreased in women submitted to IVF treatment and is associated with the presence of antiphospholipid antibodies. These factors might contribute to the increased oxidative status found in these patients.