A rotating parallel hole collimator has been designed and built for high resolution gamma camera imaging of medium energy radionuclides. This cast lead collimator has 2.0 mm thick septa and was designed for imaging 50-100 mCi I-131 radiolabeled monoclonal antibodies being administered intratumorally for brain tumor therapy. Projection data from different collimator rotation positions are summed to create a composite projection dataset with more dense spatial sampling than projection data at a single position. Measured single photon emission computed tomography (SPECT) resolution with 1-131 was 5.5-7.5 mm full-width at half-maximum (FWHM) for a 13 cm radius of rotation compared with 9.913.9 mm with a conventional medium energy collimator. SPECT resolution for the rotating collimator was only slightly worse with 1-131 than with Tc-99m, indicating good attenuation by the septa. Sensitivity was 56 counts/sec/mCi with a tight region of interest (ROI) around the projection of an 1-131 line source 12 cm from the collimator surface and 64 counts/sec/mCi with a tight ROI around the projection of an 11 cm diameter 1-131 disk source 10 cm from the collimator surface. SPECT studies of 3.7 cm and 6.2 cm diameter shellcore tumor phantoms filled with 1-131 were made. The lower activity core and higher activity shell were well resolved in a SPECT study using the rotating hole collimator. SPECT shell to core activity concentration ratios were more accurate with the rotating collimator than with a medium energy collimator. These results show the potential of this rotating collimator for high resolution human tumor studies with 1-131 tracers. © 1998 IEEE.