The Trophic Status Model of Ontario's Lakeshore Capacity Study is a refinement of the Dillon-Rigler (1975) model for predicting total phosphorus concentration and several trophic status indicators in lakes. A recent version of this model allows the modeling of watersheds, extending 3 lakes upstream for up to 15 catchments of a study lake. This feature enables lake managers to predict the effect of lakeshore development throughout an entire watershed, including downstream lakes. Differences between modeled and measured concentrations of total phosphorus for the ice-free period were less than 20% in 11 of a set of 15 lakes in a south-central Ontario watershed. Annual, seasonal and long-term coefficients of variation for measured phosphorus concentrations were 19–22% over a 9-year period in an intensively studied lake. Assumptions and sources of error in modeled and measured phosphorus concentrations are discussed. The ability of the Trophic Status Model to predict phosphorus concentrations in the absence of development is discussed and used to establish water quality objectives for total phosphorus on an individual lake basis.