To obtain the potential range in disease development, a progressive increase in spore concentration of the imperfect state ofCochliobolus miyabeanuswas used on twelve rice varieties differing in their resistance to brown leaf spot. An analysis of the relative contributory effect of increase in lesion size and numbers on the percentage leaf area diseased was used as a basis to develop a quantitative scoring method in chart form for brown spot disease. The chart was aided by the provision of diagrams of the different categories in disease intensity. The usefulness of this disease scoring chart and its limitations are discussed. Other methods of assessment are proposed to overcome the limitations especially in respect of varietal resistance and seedling blights.