The relationship between seed size and fitness in plants may depend on offspring density, especially in cases where seed size affects the outcome of competition. We investigated the relationship between seed mass, germination, intraspecific competition and seedling height in a glasshouse experiment on three European white oak species (Quercus robur, Q. petraea, Q. pubescens). Within offspring families, seed mass showed a moderate, but statistically significant effect on seedling height, i.e. seedlings from heavier seeds were slightly taller. In contrast, competition caused pronounced inequality in seedling height in pairs of competing seedlings, but in only 55.2% of all pairs the dominant competitor arose from the heavier seed. It is thus possible that a positive effect of seed mass on seedling growth can be mediated through the density of conspecific seedlings and that heterogeneity in offspring density will contribute to the maintenance of seed mass variation in oak populations.