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We evaluated interobserver reproducibility for the response evaluation criteria in solid tumors (RECIST) guidelines and the influence of minimum lesion size (MLS) on reproducibility. The 110 consecutive patients with non-small cell lung cancer were treated with platinum-based chemotherapy. Five observers measured target lesions according to both the World Health Organization (WHO) criteria and RECIST. The percentage changes for unidimensional measurements (UD; RECIST type) and bidimensional measurements (BD; WHO type) were calculated for each patient. Interobserver reproducibility among five observers, that is 10 pairs, was expressed as the Spearman's correlation coefficient for the percentage changes, the proportion of agreement and the kappa statistics for response categories. The same analysis was carried out using MLS. BD was more reproducible than UD (Spearman rank correlation coefficient, 0.84 vs 0.81; proportion of agreement, 84.4% vs 82.5%; kappa value, 0.69 vs 0.61). When MLS was applied to UD, eligible cases decreased by 6.4% and the number of target lesions by 44.6%, whereas interobserver reproducibility for UD improved (Spearman rank correlation coefficient, 0.81–0.84; proportion of agreement, 82.5–84.2%; kappa value, 0.61–0.65). The introduction of MLS to UD could also improve intercriteria reproducibility between WHO and RECIST. It is important to apply the MLS when using RECIST for the comparable interobserver reproducibility attained with WHO. (Cancer Sci 2006; 97: 214–218)