Observed diameter distributions of forest stands are adapted to the Johnson Sbprobability function. The stands investigated are untreated mixed stands of Norway spruce(Picea abies(L.) Karst.) and birch(Betula pendulaRoth andBetula pubescensEhrh.) aged between 20 and 32 years. The adaptation of the JohnsonSbprobability function is made both on mixed spruce and birch, and on each species separately. Altogether 156 observations were tested with observed distributions against calculated distributions in the Kolmo‐gorov‐Smirnov test. The fractiles are predicted with multiple regression and two multivari‐ate techniques, simultaneous‐equation models (multivariate regression) and partial least squares with latent variables. The independent variables are characteristics of site and stand. Both multivariate methods predict diameter distribution well when tested.