To assess the treatment effect of pancreatic cancer, thallium 201 chloride (201TlCl) SPECT was performed in three patients. All of the patients before and after treatment showed the same change between the tumor/liver uptake ratio of 201TlCl and the serum level of CA 19-9. In one of the patients, tumor shrinkage was not observed on CT. However, the uptake ratio decreased, accompanying with a reduced serum level of CA 19-9. These results indicate that 201TlCl SPECT is useful to evaluate therapeutic effect of pancreatic cancer.