Objective The UK’s impending departure (‘Brexit’) from the European Union may lead to restrictions on the immigration of scientists and medical personnel to the UK. We examined how many senior scientists and clinicians were from other countries, particularly from Europe, in two time periods. Design Cross-sectional study. Setting United Kingdom. Participants Individuals who had been elected as Fellows of the Royal Society or of the Academy of Medical Sciences, and UK medical doctors currently practising and listed in the Medical Register for 2015. Main outcome measures Percentages of Fellows of the Royal Society, Fellows of the Academy of Medical Sciences and UK medical doctors by nationality (UK and Irish: UKI, European: EUR and rest of world: RoW) over time. Fellows of the Royal Society and the Academy of Medical Sciences proportions were assessed for two time periods, and doctors over decades of qualification (<1960s to 2010s). Results Percentages of European Fellows of the Royal Society increased from 0.8% (1952–1992) (the year the UK signed the Maastricht treaty) to 4.3% (1993–2015). For Fellows of the Academy of Medical Sciences, percentages increased from 2.6% (pre-1992) to 8.9% (post-1992) (for both, p < 0.001). In the 1970s, only 6% of doctors were trained in the EU; the proportion increased to 11% in the last two decades (also p < 0.001). Europeans replaced South Asians as the main immigrant group. Among these, doctors from the Czech Republic, Greece, Poland and Romania made the largest contribution. Conclusions Any post-Brexit restriction on the ability of the UK to attract European researchers and medical doctors may have serious implications for the UK’s science leadership globally and healthcare provision locally.