We describe a patient with a long history of psoriasis who developed severe erythrodermic psoriasis associated with lethargy, muscular weakness and collapse. Serum biochemical screening at the time revealed severe hypophosphataemia, and when this was corrected by intravenous phosphate replacement her symptoms resolved and her psoriasis improved. Hypophosphataemia may therefore be another metabolic complication of erythrodermic psoriasis.