This paper seeks to explain the variable outcomes from university entrepreneurial ecosystems by observing their structural and spatial configurations in relation to spinoff company development. Four UK university entrepreneurial ecosystems are examined with data collected through interviews with the core actors of university entrepreneurial ecosystems: technology transfer officers, academic founders, external entrepreneurs, investors, and business incubators. It is found that university entrepreneurial ecosystem outcomes are dependent on the processes of connectedness and filtration, underpinned by geography. The effectiveness of these processes is dependent on university entrepreneurial ecosystem calibration, leading to different outcomes in terms of spinoff company formation and survival across the spectrum of universities. Successful university entrepreneurial ecosystems are characterised by strong connectedness and effective filtration, having a strong local and interregional character.