According to the findings that long, thin, and durable fibres have a high carcinogenic potency after intrapleural and intraperitoneal administration, the elongated shape of a particle represents a carcinogenic agent; this physical phenomenon is a special cause of cancer. It induces a biological process which can lead to cancer by several as yet unknown steps. However, the properties of the material the fibres are made of determine the carcinogenic potency of a fibre in a secondary way although they do not seem to be responsible for the true carcinogenic agent. For example, these properties determine the degree of solubility and flexibility. The persistence of fibres in the tissue is a very important property with regard to their carcinogenic effect because the formation of a tumour takes many years or some decades. It can be assumed that a fibre has to remain by the bronchial or serosa tissue until the induction of tumour cells occurs. If this hypothesis is correct, there could be a "durability threshold value" for fibres whose length and diameter would otherwise indicate a high carcinogenic potency. There are indications that other fibre properties apart from length, diameter and durability are important for tumour induction, however, at present, they cannot be included in a definition of carcinogenic fibres. It is proposed to classify all natural and man-made mineral fibres with an aspect ratio of greater than 5:1 as carcinogenic when they are longer than 3 microns, thinner than 1 micron (or can split into such fine fibres) and when they can persist in the tissue for more than 3 years.