When two superconductors are connected by a weak link, a supercurrent hows, the magnitude of which is determined by the difference in the macroscopic quantum phases of the superconductors. This phenomenon was discovered by Josephson(1) for the case of a weak link formed by a thin tunnel barrier: the supercurrent, I, is related to the phase difference, phi, through the Josephson current-phase relation, I = I(c)sin phi with I-c being the critical current which depends on the properties of the weak link A similar relation holds for weak links consisting of a normal metal, a semiconductor or a constriction(2). In all cases, the phase difference is zero when no supercurrent flows through the junction, and increases monotonically with increasing supercurrent until the critical current is reached. Here we use nanolithography techniques to fabricate a Josephson junction with a normal-metal weak link in which we have direct access to the microscopic current-carrying electronic states inside the link. We find that the fundamental Josephson relation can be changed from I = I(c)sin phi to I = I(c)sin(phi + pi)-that is, a pi-junction-by controlling the energy distribution of the current-carrying states in the normal metal. This fundamental change in the way these Josephson junctions behave has potential implications for their use in superconducting electronics as well as in (quantum) logic circuits based on superconductors.