Organochlorine residues in marine environmental test materials have been included in QUASIMEME Laboratory Performance Studies since 1996. The methods of data assessment include the Cofino model, which provides a sound basis for obtaining a reliable assigned value from the wide range of data distributions that occur in these studies. Both numeric and left-censored values are included in the data assessment. We review all data from the 1996¿2004 studies by determinand, assessment category (viz. satisfactory, questionable, and unsatisfactory) and laboratory. The Quality Performance Indicators developed show the collective improvements made through the sustained implementation of these studies. We discuss sources of error and proposals for further improvement.