This paper is concerned with the description of an aetiological model of anorexia nervosa and one form of bulimia nervosa. The model has been generated within the conceptual framework of 'functional analysis' and is based on a combination of research evidence and clinical observation. The model attempts to explain the development of anorexia nervosa in terms of a set of hypothesized antecedent events/variables and its maintenance/exacerbation in terms of both positive and negative reinforcers. Specifically it is suggested that initial dieting behaviour is triggered by apparently innocuous psychosocial stimuli, in the context of major setting conditions which predispose the individual towards self- and bodily control: an that successful dieting is then powerfully reinforced by its consequences, which rapidly lead to a downward spiralling into full-blown anorexia nervosa. In this formulation anorexia nervosa is viewed as an attempted adaptive strategy (which has functional value to the individual), given the major setting conditions of the individual's current life situation. Similarly, bulimia nervosa following on from anorexia nervosa is viewed as an attempted adaptive strategy given prevailing environmental and biological pressures towards increasing food intake. The model is presented in the form of diagrammatic formulations and its theoretical and therapeutic implications discussed in relation to the relevant literature..