Identity-by-descent (IBD) matrices are used for a number of practical applications, e.g. QTL-detection, marker assisted selection in breeding schemes (MAS), refining of covariances among relatives, and MAS for maintaining genetic variation. The calculation of IBD matrices can be made using Markov Chain Monte Carlo (MCMC). However, this is a computationally expensive method. Therefore, a simple deterministic method (Det) has been developed (Pong-Wong et al., 2001). The objective of this study is to evaluate this deterministic method relative to MCMC for the precision of the matrices and their performance in interval mapping and MAS.