Although hypercalcaemia is frequently associated with malignancy, it is very rare in small cell lung cancer despite the high incidence of lytic bone metastases. We report a patient with extensive small cell cancer who presented with hypercalcaemia. Investigations suggested parathyroid hormone (PTH) receptor stimulation, although the serum PTH level was not elevated. PTH related protein (PTHrP) was localised in a biopsy specimen from the tumour. Although hypercalcaemia is rare in small cell lung cancer, when hypercalcaemia does occur, PTHrP may be a causal factor.