The problem of localization of chromosomes in relation to each other in the interphase nucleus of human lymphocytes was investigated by analysis of chromatid and chromosome aberrations observed in lymphocyte cultures of three patients with Fanconi's anemia, one patient with Bloom's syndrome, and in Trenimon-treated (Trenimon, Bayer) normal cells. Distribution of open gaps and breaks is highly correlated with chromosome length and distribution of breaks involved in chromatid translocations in Fanconi's anemia and in Trenimon-treated cells. Both correlations are much lower in Bloom's syndrome. In Fanconi's anemia and in normal cells after Trenimon-treatment, the majority of chromatid translocations are between nonhomologous chromosomes, whereas in Bloom's syndrome mainly homologous chromosomes are involved. Statistical localization of chromosomes in relation to each other in the three-dimensional space by multidimensional scaling gives results consistent with the limited amount of independent evidence.