Today, laboratories involved in national or international marine monitoring programmes are strongly encouraged to seek accreditation or another formal proof of competence, The question often asked is whether a laboratory actually produces more reliable data when a quality system has been set up. A method of monitoring the differences in laboratory performance from year to year is presented in this paper. In the case of the SMHI Oceanographical Laboratory there has been substantial improvement in laboratory performance since formal quality assurance measures were introduced. (C) 1997 Elsevier Science Ltd. All rights reserved.