A simple mass balance model, combined with detailed hydrologic and mass balance measurements, yielded accurate predictions of total phosphorus (TP) concentrations in two lakes with no shoreline development. On the other hand, the model based on only natural TP loads predicted TP concentrations lower than observed in four developed lakes; however, in three of the four lakes, predictions and observations were virtually identical if the potential TP contribution from sewage disposal systems associated with shoreline development was included in the budgets. In the exception Harp Lake, the only one of the four with thick deposits of till/soil in its catchment, predicted and observed trophic status were comparable if about a third of the potential TP input from shoreline development reached the lake. The suggestion is that in this case the till/soil was effective in retaining a significant portion of TP transported from septic systems. Lakes with oxic and anoxic hypolimnia were distinguished by using different apparent settling velocities in the TP mass balance model. A lower settling velocity in those lakes with anoxic hypolimnia is consistent with TP release from the sediments under these conditions.