Let (a, b, c) be a primitive Pythagorean triple such that a = u 2 − v 2 , b = 2uv, c = u 2 + v 2 , where u, v are positive integers satisfying u > v, gcd(u, v) = 1 and 2 | uv. In 1956, L. Jeśmanowicz conjectured that the equation (an) x +(bn) y = (cn) z has only the positive integer solutions (x, y, z, n) = (2, 2, 2, m), where m is an arbitrary positive integer. A positive integer solution (x, y, z, n) of the equation is called exceptional if (x, y, z) = (2, 2, 2) and n > 1. In this paper we prove the following results: (i) The equation has no positive integer solutions (x, y, z, n) which satisfy x = y, y > z and n > 1. (ii) If (x, y, z, n) is an exceptional solution of the question, then either y > z > x or x > z > y. (iii) If u = 2 r , v = 2 r − 1, where r is a positive integer, then the equation has no exceptional solutions (x, y, z, n) with y > z > x. In particular, if 2 r − 1 is an odd prime, then the equation has no exceptional solutions. The last result means Jeśmanowicz conjecture is true in this case.