The most important mutation associated with Multiple Endocrine Neoplasia type 2B (MEN 2B) is the change of thymine to cytosine in codon 918 of exon 16 in the RET oncogene (ATG → ACG). The aim of this work was to develop a single oligoarray by using tandem hybridization to detect the T918C/RET mutation for MEN 2B patients. Two genetically non-related families were studied; each family had a member affected by MEN2B. Both patients presented the T918C/RET mutation in a heterozygous fashion. None of the relatives was positive for this mutation; thus, these cases arose de novo. The proper mutation was confirmed by with different tools, PCR-Fok I endonuclease, direct sequencing, and also using our oligoarray. In this case, it is suitable to use a DNA target smaller than 150 bases with single- or double-stranded DNA and short probes of 7-mer. It was also possible to detect the mutation by employing different sources of DNA, fresh or paraffin-embedded tissues. Therefore, the present oligoarray can identify the most common M918T mutation of RET oncogene from a variety of DNA sources with good specificity and be a good alternative in the molecular diagnosis for MEN 2B cases.