The creation of a complex web site is a thorny problem in user interface design. In this paper we explore the notion of adaptiveweb sites: sites that semi-automatically improve their organization and presentation by learning from visitor access patterns. It is easy to imagine and implementweb sites that offer shortcuts to popular pages. Are more sophisticated adaptiveweb sites feasible? What degree of automation can weachieve? To address the questions above, we describe the design space of adaptiveweb sites and consider a case study: the problem of synthesizing new index pages that facilitate navigation of a web site. We presentthePageGather algorithm, which automatically identifies candidate link sets to include in index pages based on user access logs. We demonstrate experimentally that PageGather outperforms the Apriori data mining algorithm on this task. In addition, we compare PageGather's link sets to pre-existing, human-authored index pages.