J.M. and H.G. are two unrelated male patients with developmental delay. Cytogenetic analysis detected a duplication of 17p11.2 in both patients. The extent of the duplicated region was determined using single copy DNA probes: cen-D17S58-D17S29-D17S258-D17S71-D17S445-+ ++D17S122-tel. Four of the six markers, D17S29, D17S258, D17S71, and D17S445, were duplicated by dosage analysis. Fluorescent in situ hybridization (FISH) analysis of H.G., using cosmids for locus D17S29, confirmed the duplication in 17p11.2. Because the deletion that causes the Smith-Magnesis syndrome involves the same region of 17p11.2 as the duplication in these patients, the mechanism may be similar to that proposed for the reciprocal deletion/duplication event observed in Hereditary Neuropathy with Liability to Pressure Palsies (HNPP) and Charcot-Marie-Tooth Type 1A disease (CMT1A).