Spontaneously increased chromosomal instability is well documented in the three autosomal recessive diseases, Fanconi's anemia (FA), Bloom's syndrome (BS), and ataxia telangiectasia (AT). Other conditions have been reported to be associated with chromosomal breakage. Some are still single observations: in Werner's syndrome only fibroblasts are affected, and systemic sclerosis may not be an inherited disease. Various aspects of FA, BS, and AT are discussed which have emerged since recent reviews have been published. The differential diagnosis in FA has become more important than it was in the past. Proven heterogeneity in FA demands definition of what to name FA and FA variants. The analysis of cancer frequencies and types in FA and AT lacks important clues. This should stimulate all of us to mutual exchange of data and creation of registries not only of patients and follow-ups, but also of characterized cell strains. A synopsis of results from cell and cytogenetic studies demonstrates similarities and differences in detail of the general phenomenon of chromosomal instability which FA, BS, and AT share. Results from biochemical studies at the DNA level together with cytogenetic findings indicate different but still undefined failures in DNA metabolism or DNA repair mechanisms due to the different genes. A new approach to analyzing the impairment of DNA repair in FA is briefly described. DNA related enzymes are produced in the cytoplasm and have to be transported to the nucleus. The subcellular distribution of topoisomerase activity was found to be unusual in three placentas of FA patients. Other DNA enzymes were distributed normally. Thus, a specific mechanism for movement of the enzyme through the nuclear membrane seems to be defective.