Sewage effluent was diverted away from Lake Horowhenua in April 1987. Catchment loads of nitrogen and phosphorus to the lake are estimated to have subsequently dropped by about 20% and 90% respectively. Lake condition in the winter has not improved (TP, TN, chlorophyll a, Secchi depth unchanged). Summer chlorophyll a, however, has halved, and it covaries significantly with TN, which has also fallen substantially. Summer TP and DRP have also more than halved, but levels remain very high (DRP 150 mg/m3, primarily because of a continuing high internal P load. Water clarity is still poor, however, because levels of inorganic suspensoids and detritus remain high. Further falls in summer chlorophyll a would seem to require major reductions (3-4 fold) in lake TP following such a drop in the internal P load then. By contrast, winter chlorophyll a should reduce in parallel with any reduction in the internal load then. The rate at which the internal P load will fall is unclear ; future monitoring will document this, and any consequent improvement in lake condition.