Abstract: In 1958, a disease process termed progressive multifocal leucoencephalopathy was described by Astrim, Mancall, and Richardson.They presented three cases and reviewed the literature, finding five more which fulfilled their criteria.Thereafter, sporadic cases were reported, and in 1961 Richardson reviewed these cases and added more from his own experience, bringing the total of documented cases to 22.He discussed the various clinical and pathological features of the disease, and described one new micro- scopic finding common to three cases, namely, alteration of the nuclei in the deeper portion of the cerebellar granular layer.Since then several new case reports have appeared, but the disease remains a rare one with a poorly understood aetiology (Hecker and Reid, 1962).Each new case deserves to be re- corded.We wish to present two additional cases.One demonstrated no symptoms during life which could be attributed with certainty to this disease.It showed scattered, small, presumably early lesions at post-mortem examination.The other case, in con- trast, demonstrated the fully developed clinical and pathological picture.CASE REPORTS CASE 1 A.M., a Caucasian woman, was transferred from a state mental institution to the Palo Alto Veterans Administration Hospital in April 1961, at the age of 82.She had been active as a registered nurse most of her adult life and had essentially no medical history until she was involved in an automobile accident in August 1959.She was briefly admitted to a hospital for observation and treatment of fractured ribs.There was no evidence of head injury.After discharge she became neglectful of her personal appearance and household, and she began to drink.She also had paranoid ideas about her neighbours and was admitted to a mental hospital later that year.At that time she was fully orientated but aware that she could not care for herself.She often became confused, thinking that her belongings had been stolen, her house raided by intruders, and imagining that something horrible was about to happen to her.In December 1960, she had a bout of pneumonia; following this she was weak, con- fused, unsteady on her feet, and her memory appeared worse.