Urinary catheters tend to block when biofilm from urease-producing organisms build up on the catheter surface. This is a locally-occurring process that influences and influenced by the composition of the urine. In this work we relate urine pH and calcium to catheter blockage and suggest how to reduce the rate of encrustation. Sixty patients with indwelling urinary catheters were studied, 26 of them being troubled by frequent blockage of their catheters, 34 of them not. A series of small urine samples were collected during a 24 h period. Urinary pH and calcium concentration were combined into discriminant functions designed to separate Blockers from Non-blockers and achieved a 95% correct classification. The results indicate that a high and uniform rate of fluid intake is mandatory for the patient with a tendency for catheter blockage. Excessive total fluid intake may be avoided by attention to uniformity. Other avoidable risk factors include: excess dietary calcium from certain protein supplements and antacids; excess dietary magnesium from certain beverages and antacids; alkali from effervescent tablets; excess dietary citrate from some fruit juices and cordials; intermittent dehydration from alcohol ingestion. Less tractable risk factors include infection of the urinary tract with urease-positive organisms, hypercalciuria of immobilisation, hyperhydrosis and postural oliguria. The processes involved in catheter encrustation and blockage provide a model for the formation of calculi in spinal cord injured patients. Therefore the above considerations may also be relevant to the management of stone disease in paraplegic and tetraplegic patients.