Patients with adenomatous polyposis syndromes such as familial adenomatous polyposis are at higher risk of colorectal cancer, hence continuous management is necessary. However, little is known about the etiology of patients with numerous laterally spreading tumors (LSTs), or how genetic alterations uniquely influence LSTs in colorectal carcinogenesis. The present case report investigated a woman with >150 non-granular type LSTs (LST-NG) and one sigmoid colon cancer. After subtotal colectomy via ileorectal anastomosis, genetic and epigenetic analyses were conducted by comparing the profiles of the patient's normal colonic mucosa, four LST-NG lesions and a cancer lesion. Using customized multigene panel testing, no pathogenic germline mutations were detected, including APC regulator of WNT signaling pathway, but identified a somatic pathogenic variant of APC in one LST-NG lesion, and both TP53 and F-box and WD repeat domain containing 7 somatic mutations in the cancer. Comprehensive genome-wide methylation analysis showed that CpG island promoters at zinc finger protein 625, LON peptidase N-terminal domain and ring finger 2, WD repeat domain 17 and syndecan 2 were methylated in both LST-NG and cancer, which may contribute to colorectal tumorigenesis as early as the LST-NG phase.