Background England, Scotland and Norway have established Criminal Cases Review Commissions to review the convictions of persons who may have been wrongly found guilty of a criminal offence. Aims The aim of this study was to explore the Norwegian Criminal Cases Review Commission (NCCRC) practice when legal insanity constitutes grounds for possible reopening of a criminal case. Methods We identified all cases between 2004 and 2012 where an application to the NCCRC to reopen a case was on grounds of concerns about insanity. Data were extracted from records. Differences between reopened and disallowed insanity cases and variables that may explain the NCCRC's decision to reopen a case were analysed using multiple logistic regression. Results Forty cases were allowed, and 34 disallowed. The NCCRC relied fully and unanimously on the conclusions of the expert reports in making these decisions. In 26 of the 40 cases that the Commission reopened, psychosis had gone undetected by the courts. Such undetected cases were most often discovered in the wake of a new criminal case in connection with the defendant. If the length of the sentence were short, it was less likely that insanity had been considered by the court in the index judgement. Conclusions The NCCRC may rely excessively on the forensic psychiatric reports. The experts' conclusions may be uncertain, given that they often have to look back several years to establish mental state and its relevance at the time of the crime. As nearly two‐thirds of the reopened insanity cases had been given penalties of under 6 months, it may be asked whether such further investigation of fairly low‐level crimes is the best use of resources. Copyright © 2015 John Wiley & Sons, Ltd.