A survey of the admissions to the Cook County Hospital, Chicago, for the five-year period from 1954 to 1958 reveals that in a total of 396,220 admissions (excluding psychiatric cases) only two cases of thallium poisoning were recognized. While thallium poisoning is not common, neither is it so rare that it should be seen only twice in 396,000 admissions to a hospital catering to the less privileged classes. This suggests that its clinical manifestations are at times confusing and that it may simulate other conditions. The suspected diagnosis can be confirmed only by biochemical analysis. It is not the purpose of this paper to review the literature regarding thallium poisoning, but rather to add two patients whose presenting sign was ataxia. We wish to point out the salient features and the possible sources of diagnostic confusion and to report on the use of dimercaprol (BAL) and trihexyphenidyl (Artane) in therapy