Evidence is growing that in many markets consumers select not simply on the basis of the perceived attributes of products, but their preferences are modified by the behaviour of others. Economists have paid relatively little attention to such markets. We consider evidence from the activity of hill-walking. The Munros are a list of Scottish hills over 3000 feet in height. We analyse completions of both the Munros themselves, and the Munro Tops, a difficult and time-consuming extension of the Munros. The classic Bass diffusion model incorporates the imitation of others as a part of the behavioural rules used by consumers in making choice. We extend the Bass model to be able to apply it to long-term case studies where substantial changes over time in the population making choices at any given point have to be taken into account. The Munros are a particular illustration of this, but the extension can be used in other situations where such population changes are important. Our results show that Top completions are dominated by the ‘fashion’ component, suggesting there was a cohort among which Tops completions became fashionable, but which has not been sustained. Both our extended model and the standard model can account for this. In contrast, our extension is needed to explain why Munro completions have remained close to their peak level for a decade now, a fact for which the standard Bass model is unable to account.