The probability of co-channel interference (PCI) due to multiple correlated log-normal signals is calculated for cellular radio systems operating in Rayleigh fading and log-normal shadowing environment. The effects on the PCI of the correlation between the signals, the standard deviation due to shadowing, the number of interferers, the co-channel protection ratio, and the traffic load is investigated. The results are used for analyzing the optimal channel reuse, the cluster size, and the spectrum efficiency in terms of these parameters.