  An n-dilation of a contraction T acting on a Hilbert space H is a unitary
dilation acting on H \oplus C^n. We show that if both defect numbers of T are
equal to n, then the closure of the numerical range of T is the intersection of
the closures of the numerical ranges of its n-dilations. We also obtain
detailed information about the geometrical properties of the numerical range of
T in case n=1.
