Our objective in this study was to determine possible associations between antiphospholipid antibodies (aPL) and histologically defined heart valve lesions in the MRL-lpr/lpr mouse, a suitable model for the antiphospholipid syndrome (APS). At monthly intervals, from 2 to 6 months of age, three MRL-lpr/lpr mice (two with anticardiolipin antibodies, one without) and two sex- and age-matched Balb/c mice (controls) were sacrificed for histological studies. Serum binding to phospholipids and DNA was studied at this time. We found thickened heart valves in 68% of MRL-lpr/lpr mice and in 80% of Balb/c mice, and no association with any of the antibodies tested was found. No evidence of coronary vasculitis or thrombi was found in any of the mice studied. Platelet counts in MRL-lpr/lpr mice were significantly lower (640.550 +/- 211.818 x 10(6)/ml) than in Balb/c mice (780.0 + 112.5 x 10(6)/ml) (p < 0.05), and no association was found between platelet counts and aPL. In this model of murine APS, aPL bear no importance in heart valve pathology.