Three cats and 1 dog that had undergone renal transplantation because of end-stage renal disease were examined because of complications 3 to 6 weeks after surgery. One cat died prior to treatment of the complications; Toxoplasma cysts were found in sections of the renal allograft, and Toxoplasma tachyzoites were found in other organs. The other 2 cats and the dog died despite treatment, and protozoal cysts, as well as tachyzoites, were identified in other organs but not within the allografts, suggesting that reactivation of latent infection following immunosuppression was the most likely cause of disseminated toxoplasmosis. These cases illustrate that toxoplasmosis can be a fatal complication in renal transplant recipients. We currently recommend that feline and canine donors and recipients undergo serologic testing for toxoplasmosis prior to surgery. In addition, we suggest that seropositive donors not be used for seronegative recipients and that seropositive recipients and that seropositive recipients be monitored closely after surgery for clinical signs of toxoplasmosis.