The purpose of this paper is to measure the technical efficiency of farms on the basis of data envelopment analysis (DEA) and to present an analytical method of farm diagnosis using measurements obtained for melon farms in Japan. The paper first describes the objectives of melon farms operating under the marketing strategy of agricultural cooperatives and explains DEA, which measures the technical efficiency of such farms. It then uses this analytical method to measure technical efficiency for each farm and demonstrates h6w technical efficiency is dependent not only on the management performance of production of melons and other crops but also on the number of full-time farm workers. In addition, the effect of management perlormance on technical efficiency is analysed quantitatively for each size group of full-time farm workers. The paper also illustrates how DEA can be used for farm diagnosis when the management performance of inefficient melon farms is compared with that of efficient farms. DEA is able to indicate some efficient farms as the optimum solution that should be treated as the model for the improved management of inefficient farms. Data are drawn from a sample of 91 melon farms in Choshi, Chiba Prefecture.