The implementation of new Capital City projects faces various uncertainties. Some of these uncertainties are understood and acted upon, but some are not. Existing social relationships in which Capital City projects are planned and implemented are less regarded as a factor underlying uncertainties of achieving plans' objectives and assumptions. These turned out to be a principal factor constituting the uncertainty of realising Abuja and the FCT plans. We believe, however, that institutional context both as a damaging and enabling factor is too important to be discounted in capital city plans. It is traced from the Nigerian urbanisation experience to traditional societies, previous policies and the administrative structure of the Federal Capital Development Authority. The review of other capital city programmes has only confirmed the need to understand the institutional framework in capital city plans. The research identified that uncertainties tend to increase with monopoly of dec ision-making by individuals in authority, lack of knowledge, failure to involve groups or bodies who should be involved, poor management, scale of projects, the time dimension and political instability. The research has itemised uncertainties which could be deduced from major planning projects, and an anlysis of which helps in policy formulation. Principal conclusions included that pluralism compounded the problems of the implementation of Capital Plans and the capital cities have to adapt and modify the institutional framework. A diversified resource base is essential in capital city development. Institutions and organisations could be employed positively to reduce uncertainties of funding and housing. Common to capital city programmes is that uncertainties have not been faced seriously by planners and decision makers as they have been in the study of psychology or in business deals. A simultaneous consideration of methods of assessing uncertainties in planning is essential in a new urban development.