The objective of this study was to apply a microscopic lesion scoring system for assessing the severity of experimental coccidiosis. To accomplish this objective, we examined the effects of varying doses of E. maxima in chicks by comparisons of gross and microscopic lesion scores, body weight gains, feed conversion ratios and faecal oocyst counts. In the battery trial, 3-day-old chicks inoculated with 2.5 x 10(3) or more oocysts had significantly lower growth rate than the controls. The mean feed conversion ratio for days 0 to 36 were significantly greater than those of 0 to 10 and 0 to 17 p.i. The intestinal gross lesion scores and microscopic lesion scores were significantly affected by days p.i. and oocyst dosage. In the floor pen trials, the mean adjusted body weights of 3- and 14-day-old chicks inoculated with 2.5 X 10(4) oocysts of the same strain of E. maxima were significantly lower than the controls. Gross and microscopic lesion scores regressed with significant (P<0.05) linearity on body weight gains. Under experimental conditions, both duodenal and jejunal microscopic lesion scores regressed with significant linearity on gross lesion scores.