The phenotype of dup(3q) syndrome partially overlaps with Brachmann-de Lange phenotype. Convulsions and eye, palate renal, and cardiac anomalies are more frequent in dup(3q) syndrome, while limb deficiencies, hirsutism, and synophrys are more characteristic of Brachmann-de Lange syndrome. Whether the two syndromes have a biological relationship has yet to be demonstrated. Using two patient translocation cell lines, each involving distal 3q, we have narrowed the critical region of the dup(3q) syndrome to the interval 3q26.31-q27.3 and initiated its molecular characterization. We have mapped in this region 6 cosmid clones spanning approximately 3-5 Mb. The critical region appears to overlap with the region where a balanced translocation was found in a Brachmann-de Lange patient. This work provides the mapping framework for finer molecular analysis of dup(3q) syndrome.