Over 1500 adenomatous polyposis coli (APC) gene mutations have already been identified as causative of familial adenomatous polyposis (FAP). However, routine genetic testing fails to detect mutations in about 10% of classic FAP cases. Recently, it has been shown that a proportion of mutation‐negative FAP cases bear molecular changes in deep intronic and regulatory sequences. In this study, we used direct sequencing, followed by multiplex ligation‐dependent probe amplification (MLPA) of genomic DNA from family members, affected by classic FAP. We first reported the family as mutation negative. With the launch of a new version of MLPA kit, we retested the family and a novel full deletion of promoter 1B was detected. The exact breakpoints of the deletion were determined by array comparative genomic hybridization (CGH) and long range polymerase chain reaction (PCR), followed by direct sequencing. The total APC expression levels were investigated by quantitative polymerase chain reaction (qPCR) assay and allele‐specific expression (ASE) analysis. The APC gene expression was highly reduced, which indicates causative relationship. We suggest that there is a significant possibility that APC promoter 1B mutations could be found in mutation‐negative FAP patients. In the light of our findings it seems reasonable to consider targeted genetic re‐analysis of APC promoter 1B region in a larger cohort of unsolved cases.