A description of a quantification performance by laboratories taking part in two intercollaborative studies (surveys 1 and 2) carried out in 1993 (n = 195) and 1994 (n = 228), respectively, on urine drug testing in the European Union is presented. The performance of laboratories that quantified at least one substance (33.8% and 34.6% out of the total laboratories in surveys 1 and 2, respectively) is reported as a function of the analytic technique applied for quantification, the number of parameters investigated, the precision and accuracy of results obtained, and the influence of availability of reference material, including deuterated drug-standard analogue solutions provided in survey 2. Quantification of different drugs of abuse was mainly carried out by gas chromatography coupled to mass spectrometry (63.1% of analyses in survey 1 vs. 68.6% in survey 2). It should be noted about the data obtained that the results for quantification were not particularly satisfactory in terms of precision and accuracy (CV mean = 62.8 in survey 1 and 50.4 in survey 2 and error % mean = 12.7 in survey 1 and 18.6 in survey 2). The provision of reference materials increased the number of substances quantified (45.3% in survey 1 vs. 51.8% in survey 2) and improved the performance (CV mean of matching laboratories when quantifying matching substances: 50.1% in survey 1 vs. 35.5% in survey 2). The impact of a "learning factor" (the opportunity to be challenged a second time and to correct past errors) and the influence of the provision of deuterated analogues in survey 2 was investigated in a population of repeater laboratories that quantified both in surveys 1 and 2 (55 vs. 63 laboratories, respectively). According to the European experience reported, it may be observed that laboratories able to afford quantitative analyses provided better overall analytic performance. This is probably because quantification implies some validation of the analytic procedure. Participation in interlaboratory comparison programs also aimed at assessing the performance of quantitative analyses should be considered a quality asset for laboratories involved in drug testing.