The number of health workers at Oxfordshire hospitals from elsewhere in the EU has risen to more than double the national average.
Over the last four years the proportion of EU migrants employed at Oxfordshire University Hospitals Trust has risen from 2% (240) to about 11% (1,400).
The figures, obtained after a Freedom of Information request, compare with 4.6% of NHS workers nationally.
The trust said it has been targeting newly-qualified nurses from the EU.
Over the same period, 2012 to 2016, the number of nurses and midwives rose from 56 to 787.
The figures come ahead of Thursday's vote on whether the UK will leave the European Union.
Anneliese Dodds, Labour MEP for the South East, said the figures were "very significant".
She added that if we left the EU and it were harder for people to come and work here it would "take a long time and be expensive" to train more people.
Ms Dodds said: "We shouldn't stop a system that seems to be working for us."
But Keith Mitchell, Conservative former county council leader, said we "shouldn't show an automatic preference to people inside the EU".
He added the most important thing was that those employed "can speak English well and have good training and good standards".
The trust said in a statement that the rise was partly down to record keeping.