Diffuse phosphorus (P) loads to a small lake, Friary Lough, in a 1 km2 agricultural subcatchment were quantified over 90 years using a palaeolimnological model. The model assumes that lake total phosphorus (TP) is lost to the sediments and to the lake outflow during periods of steady-state or is also stored within the water column during periods of non-steady-state behavior. Reconstructed TP loads during the 1991-1995 time interval of 2.05-2.53 g m-2 yr-1 are verified by hydrochemical monitoring results from the lake inflow during 1997-1998. This provides evidence for the accuracy of the palaeolomnological model and also that TP loads to the lake can be accounted for from external catchment runoff. An analysis of the TP load data in terms of catchment exports shows that there was a linear rate of increase from ca. 1946 to 1995 of 1.20-1.56 kg km-2 yr-1. The rate of increase is similar to river P load data in the larger 1480 km2 catchment taken over 17 years. The rate of TP increase to the lake is interpreted with regard to current soil P models that propose increasing and threshold soil P concentrations as the cause for increasing diffuse P loss in runoff.