This paper presents a comprehensive study of the influence of environmental uncertainty on business-IT alignment. The existing literature postulates environmental uncertainty as a key challenge to achieving business-IT alignment. Hence, the first objective of this study is to identify the extent of the impact of environmental uncertainty on business-IT alignment, and to determine its relative impact in the light of the other antecedents. Furthermore, small and medium sized enterprises (SMEs) differ fundamentally from large firms in many ways. Thus this paper also aims to investigate the variation between SMEs and large firms with regard to the antecedents for strategic alignment. Based on data collected from 212 firms, a conceptual model is tested against the research objectives. The findings provide important contributions to both research and practice by demonstrating the relative impact of environmental uncertainty, and showing how the antecedents of alignment vary between SMEs and large firms.