Adequate treatment of a disease process involves an under standing of the natural history of the disease. It is, of course, highly desirable that the aetiology should also be known and it is useful if terminology is sensible and universally accepted. Ignorance of aetiology and variability of nomenclature, however, are not bars to successful treatment provided that in any given instance the evolution of a disease is fully understood. This lecture is concerned with a group of conditions of fibro blastic origin which are known collectively as the fibromatoses. Their aetiology is unknown, but their great importance lies, firstly, in the fact that they are often misdiagnosed as true malig nant neoplasms and, secondly, in the sad truth that all too frequently they are very badly treated. Firstly, then, I shall try to define what I mean by a fibro matosis. Secondly, I shall list those conditions which seem to me to justify the label, and, thirdly, I shall deal briefly with selected examples. Anyone interested in tumours and tumourous conditions of soft tissues knows all too well the vagaries of mesenchymal be haviour, and it is the fibroblastic line of differentiation that I am concerned with here. Terms such as inflammatory, reactive, or neoplastic while appropriate and accurate in many instances are inadequate to define the behaviour patterns of all fibroblastic proliferations. At least one more category is essential, hence the fibromatoses. This concept has been developed largely since the second world war, and most of the main papers have been in the American literature. Nevertheless, the existence of this strange group of fibroblastic disorders has been known for many years, and Janssen (quoted by Touraine and Ruel1) wrote of the diath?se fibroblast?que as long ago as 1902.