[en] Early stages of hydration in a sodium iodide crystal may cause artifactual ''hot'' spots on thallium-201 heart images. This phenomenon was observed when a mobile gamma camera was used even though the daily quality-control flood-field views revealed no apparent abnormalities. Nevertheless, hot spots were visible on thallium-201 and xenon-133 flood-field images. With asymmetric photopeak positioning using a technetium-99m source, hot spots were noted in addition to phototube nonuniformities. Subsequently, the camera head was disassembled, the crystal was inspected visually, and the crystal seal was found to be broken. Hydration of the crystal was confirmed by the camera manufacturer. Quality-control procedures should test for hydration-induced artifacts on a regular basis