We present a statistical method to derive the mass functions of open clusters using sky survey data such as the 2 Micron All Sky Survey (2MASS) and the Guide Star Catalogue (GSC). We have used this method to derive the mass functions in the stellar/substellar regime of three young, nearby open clusters, namely IC 348, σ Orionis and Pleiades. The mass function in the low mass range (M< 0.50M
⊙) is appreciably flatter than the stellar Salpeter function for all three open clusters. The contribution of objects below 0.5 M⊙ to the total mass of the cluster is ~40% and the contribution of objects below 0.08 M⊙ to the total is ~4%.