Given a fixed elliptic curve E
defined over Q having no rational torsion points, we discuss the probability that the
number of points on Emodp is prime as the prime p varies. We give conjectural
asymptotic formulas for the number of p ≤ n for which this number is prime, both in
the case of a complex multiplication and a non-CM curve E. Numerical evidence is
given supporting these formulas.