Genetic variation in the durability of Pinus sylvestris L. heartwood to a brown rot fungus, Coniophora puteana, was studied using an in vitro decay test. Juvenile heartwood was sampled from 33-yr-old half-sib families growing in a progeny test and from their mothers in a clonal archive. The narrow-sense heritability for the heartwood weight loss was 0.37, and the coefficient of additive genetic variation was 28%. Heritability estimated by the regression of the offspring on mothers was 0.29, and the coefficient of genetic prediction was 0.24. These results indicated that it would be possible to improve the decay resistance of P. sylvestris heartwood by direct selection. According to the genetic correlation (rA = 0.36), selecting for heartwood density would result in an unfavourable response in weight loss caused by C. puteana. However, it appears that unknown environmental factors, which increase heartwood density, also decrease the heartwood weight loss (rE = -0.56). This result emphasizes the need for better understanding of the relationships among wood density, decay fungi, and environmental factors.