The yield of chromosome aberrations (dicentric chromosomes, ring chromosomes) induced in lymphocytes of four radiotherapy patients was determined. In each case this yield was drastically increased when compared with the spontaneous incidence, the dicentric frequencies increasing by the following factors: 604, 525, 600, and 725. With the aid of the dicentric yield obtained, the following equivalent whole body doses were estimated ("biological dosimetry"): 309, 286, 308, and 341 rd. Comparing the observed with the theoretical number of cells with 0, 1, 2, 3 ... dicentrics ("distribution analysis"), we found a highly significant departure from the Poisson distribution, as was to be expected after partial body irradiation. The distribution analysis seems principally suited to discriminate between partial and whole body irradiation.