After a prolonged period of chemotherapy for a disseminated nodular lymphoma, a 51-year-old woman developed a white lesion in the left macular region, in the absence of any overt lymphomatous disease elsewhere. The initial retinal diagnosis was nonetheless a lymphomatous infiltrate. Due to her severely immunocompromised state, the patient developed pneumonia and died. The involved left eye was examined postmortem by light and electron microscopy. The retinal lesion was discovered to be caused by infection with Toxoplasma gondii, which on postmortem evaluation was also found in the brain together with subclinical lymphomatous disease and multifocal leucoencelphalopathy. Many viable and intact cysts were observed floating within the necrotic retina as well as lodged within recognizable retinal cells. Most of the organisms were encysted bradyzoites, although a rare tachyzoite was discovered in the extracellular space. Unusual features examined ultrastructurally were a preretinal membrane formed exclusively by glial cells, residual proliferating glial cells within the retina that were frequently related to surviving capillaries, and a multilaminar spindle cell placoid proliferation of pigment epithelial cells, assuming almost pseudo-sarcomatous proportions. Banded extracellular membrane material was focally observed between the proliferating pigment epithelial cells. No neoplastic lymphoreticular cells were discovered within the choroid, but a curious feature was the production of basement membranes by the surviving choroidal melanocytes. Toxoplasmosis should therefore be considered along with fungi and viruses in the differential diagnosis of white retinal lesions appearing in immunocompromised patients.