We have attempted to define a normal range for blood urea and creatinine for elderly inpatients and to determine the relative importance of pre-renal, renal and post-renal pathology in those with renal impairment. A total of 118 admissions to an acute geriatric unit and 67 separate post mortems in patients over 67 years of age were studied prospectively. Up to 123 items of data were coded and analysed including blood urea and creatinine, clinical or pathological changes associated with renal disease, clinical outcome and post mortem findings. We determined our own 'normal' hospital ranges for urea (1.4-13.2 mmol/l) and creatinine (48-141 mumol/l) from plasma values in 76 patients with no evidence of renal impairment, either on admission or in the past. Using these values 41% of post mortem cases and 25% of clinical admissions had a raised blood urea. Pre-renal conditions such as cardiac failure, dehydration and gastrointestinal haemorrhage, either alone or in combination, were present in 56% of these patients. Urea and creatinine values were substantially higher in patients who died in hospital as opposed to those who were discharged or transferred. Creatinine values were greater in those with intrinsic renal disease or post-renal obstruction as compared to patients with pre-renal causes of renal impairment. Patients with histological evidence of extensive glomerulosclerosis or nephrosclerosis had higher urea and creatinine levels than those with only minor ageing changes.