In order to relate histopathological findings of the kidney in systemic vasculitis to renal outcome, scoring of various morphological parameters is necessary. Therefore, we conducted a standardization study for evaluating renal biopsies from patients with systemic vasculitis. Four experienced renal pathologists from four European centres joined in the study. A scoring protocol was devised that required the observers to score an extensive number of histopathological lesions either quantitatively (as a percentage of the total number of glomeruli) or dichotomously (on a present/absent scale). Twenty renal biopsies were scored individually by all the observers, from which the inter-observer variability was analysed. Ten randomly chosen biopsies were scored again, in order to obtain the intra-observer variability. For inter-observer agreement, the evaluation of the quantitative variables was satisfactory for both rounds (0.55 < or = Kendall's W < or = 0.95 and 0.59 < or = W < or = 0.96, respectively, with all P < 0.05). However, the inter-observer agreement for the dichotomous data was poor (kappa < or = 0.30 in more than half of the parameters in both rounds). Also the data on intra-observer agreement showed more favourable results for the analysis of the quantitative data (Pearson's r > 0.45 in more than 85% of the variables in both rounds) than for the dichotomous scoring system (kappa < or = 0.30 in more than half of the variables). It is concluded that even between experienced renal pathologists discrepancies occur in scoring kidney biopsies. Inter- and intra-observer agreement is greater if a quantitative method for reviewing the biopsies is applied that requires the observers to score the tissue specimens systematically.