Let f and g be both continuous functions on (0, ∞) with g (t) > 0 for t ∈ (0, ∞) and let F (x) = L (f ), G (x) = L (g) be respectively the Laplace transforms of f and g converging for x > 0. We prove that if there is a t * ∈ (0, ∞) such that f /g is strictly increasing on (0, t * ) and strictly decreasing on (t * , ∞), then the ratio F/G is decreasing on (0, ∞) if and only if ∞ 0 e −x cosh t cosh (vt) dt, 2010 Mathematics Subject Classification. Primary 44A10, 26A48; Secondary 33B15, 33C10. Key words and phrases. Laplace transform, monotonicity rule, psi function, modified Bessel functions of the second kind.This paper is in final form and no version of it will be submitted for publication elsewhere.