Thirteen phenotypically abnormal patients with previously identified de novo or familial, apparently balanced, chromosome rearrangements were reexamined with high-resolution techniques. No definite imbalance could be demonstrated in any of the cases. However, some breakpoints were reassigned to more specific sub-bands and others to totally different bands. The study confirmed translocation reciprocity in some cases in which metaphase banding techniques failed to allow such determination. In one patient an apparent extra dark band was observed which could be explained by limited uncoiling, intraband exchange or small band duplication. In two patients limited uncoiling was observed in one derivative chromosome. Tissue-limited mosaicism was discovered in cultured fibroblasts from one of the seven patients evaluated.