Three different systems for quantitative coronary analysis [Cardiovascular Measurement System (CMS); Polytron 1000; Angiographic Workstation (AWOS)] were compared in 109 patients before and after coronary angioplasty and at follow-up coronary angiography. Correlation coefficients were low and 95% limits of agreement were wide. In general, CMS exhibited a tendency to yield lower values for very small diameters and higher values for larger vessels. The acute gain in minimal luminal diameter was considerably smaller when assessed by AWOS as compared to Polytron (0.52 vs. 0.71 mm, P < .0001) and to CMS (0.52 vs. 0.75 mm, P < .0001). Long-term gain was much larger when assessed by Polytron as compared to AWOS (18.8 vs. 11.5%, P < .001) and it was almost double for CMS as compared to AWOS (20.7 vs. 11.5%, P < .0001). In conclusion, in the individual patient very different results can be obtained when different QCA systems are used, and systematic differences between the systems are encountered.