DR. JOHNSON RESPONDS: Recent press interest in mad cow disease in Europe has been confusing to patients' families and, in some cases, their physicians — so I can understand your concern. It is true that since 1994 a variant of CJD related biologically to mad cow disease has appeared in Britain, where more than 150 cases have been recorded. The only U.S. case was a woman who died in Florida, but she had been raised and exposed in England. No patient in the U.S. has yet been found with this variant CJD, which has different clinical symptoms and pathology from other CJD types. Several hundred patients die every year in the U.S. from the common form that occurs sporadically with no known source of transmission, but these cases are caused by a different prion protein than mad cow disease and variant CJD. The cases of sporadic CJD have not been related to eating meat; indeed, this disease form, which I presume the correspondent's relative had, occurs at a rate of one per million persons annually worldwide. It occurs in vegetarians and in societies that do not eat beef. Whether this disorder occurs spontaneously or is acquired from some environmental source remains a major mystery in medicine.