The thesis is presented that progression, in both neoplastic and nonneoplastic disorders, involves a common biological mechanism. This view derives from the author's unified theory of growth and age-dependent disease. Diseases that show a reproducible age-dependence, satisfying certain statistical criteria, are held to be initiated by random 'errors'--somatic gene mutations--in stem cells of the central system of growth-control. A specifically mutant stem cell propagates a "forbidden clone' (Burnet) of descendant cells and these cells, or their secreted humoral products, attack target cells at one or more anatomical locations. In certain disorders, a single forbidden clone gives rise to the first detectable symptoms and signs but progression of the disease depends on the (random) formation of further forbidden clones that attack additional target cells. This theory is discussed with reference to the neoplastic disorders, familial intestinal polyposis and tumours of the uterine cervix; and the non-neoplastic disorders, Parkinsonism and male baldness. Related problems concerning the fatal crisis in malignant disease and the unifocal/multifocal origin of tumours are also discussed.