Subdural temperature recordings under hardening methyl-methacrylate cranioplasties of varying sizes but standard thickness were analyzed. If 29.7 degrees C saline irrigations are used during hardening, the subdural temperature rises no more than 0.55 degrees C. The conclusion is that letting methyl-methacrylate harden in situ during cranioplasty is safe provided 29.7 degrees C saline irrigation is utilized.