To the Editor.— If indeed Alker et al have had "Long-Term Experience with Isotope Cisternography," (219:1005,1972), they should be aware that "allergy to isotope" is an unknown entity in nuclear medicine procedures. One hundred microcuries of131I would contain.00000081 mg of iodine (8.1×10-8mg). Normal cerebrospinal fluid contains 0.2μg/100 ml to 18μg/100 ml. The total volume is 100 to 150 ml. During the performance of radioisotope cisternography, one is then adding three orders of magnitude less than the iodine that normally exists. This, of course, does not eliminate the possibility of iodine hypersensitivity, but to reiterate, there is no documented case of hypersensitivity reaction to administered radioisotope. If one complies with the regulation of administering less than 4 mg of albumin in toto, the incidence of adverse effects from this very useful diagnostic procedure is negligible. The same principles apply to radioisotopic studies of the kidneys. Hypersensitivity