Variance behaviour and correlation structures in ion beam induced cascades have been studied by Monte Carlo method. It was found that the number of defects created in an infinite medium has an over-Poisson variance due to fluctuations in the electronic losses. The defect fluctuations are enhanced in the case of a slab geometry due fluctuations of escaping particles and their loss of progeny. The structure of the vacancy depth correlations supports the variance results and indicates clustering effects in an individual cascade.