Transactions entered into by persons suffering from periodic insanity during intervals in which there is uncertainty as to whether they were in a state of insanity or lucidity have consistently been regarded as one of the most challenging issues in Iranian jurisprudence and law. This study, employing a descriptive–analytical method and a comparative approach, examines the legal status of such transactions in Iranian law, with particular reference to judicial precedents, divergent doctrinal views, and juristic opinions, as well as in the legal system of England. The findings indicate that Iranian courts have issued considerably divergent rulings regarding the validity of transactions concluded by periodically insane persons. While some judgments have held such transactions to be void, others have recognized their validity when concluded during periods of lucidity. Nevertheless, the Supreme Court of Iran, through a judicial decision, has presumed the continuance of insanity and, relying on the doctrine of Istishab, has analyzed this category of transactions accordingly. Legal doctrine has likewise advanced differing viewpoints on this issue, ranging from acceptance of the presumption of continuing insanity to the requirement of establishing the individual's state of awareness and lucidity at the time of the transaction. In the legal system of England, a more flexible approach has been adopted under the doctrine of voidable contracts, whereby the matter is assessed from a psychiatric and case-specific perspective. Rather than relying on presumptions of continuity, English law considers the individual's mental capacity at the precise moment of contract formation as the decisive criterion.