In this paper, a proactive decode-and-forward cognitive relay network is considered with a maximum transmit power limit and underlay spectrum sharing. The outage probability for the secondary system is derived for Rayleigh fading channels with and without primary user interference. This analysis is validated by simulation. The results obtained show that both the maximum transmit power limit and the interference power constraint cause outage saturation. Interference from the primary transmitter degrades performance, but an increase in the number of secondary system relays can improve performance.