A method and apparatus for monitoring the operability of components in a hemodialysis apparatus during dialysis treatment. The dialyzer is isolated from the dialyzing liquid circuit for a time interval during dialysis treatment. During the isolation time interval, the dialyzing liquid circuit pressure is measured and monitored to detect deviations from a mean dialyzate operating pressure. The isolation is repeated periodically, with the time period between isolation intervals dependent upon the ultrafiltration coefficient of the dialyzer. This method may be performed automatically by a microprocessor in the hemodialysis apparatus.