Cranial hyperostosis is a common secondary manifestation of intracranial meningiomas. This may occur with or without neoplastic invasion, apart or even remote from the growth. Alkaline phosphatase (AP) is one of many enzymes produced by meningiomas and is known to possess indirect ossifying properties. Meningiomatous cranial hyperostosis could possibly be mediated by a humoral mechanism. This hypothesis was tested using chemical and histochemical determination of the occurrence of the enzyme in a series of hyperostosing and non-hyperostosing meningiomas. In the hyperostosing type the content of AP was in average more than three times as high as in the non-hyperostosing type. The results thus are in favour of the induction hypothesis.