Method for (i) determining the prognosis of a colon or colorectal cancer subject after surgery or resection removing the primary tumor and prior to a treatment following the surgery or resection of the primary tumor, (ii) determining the need for additional medical treatment for a colon or colorectal cancer subject after treatment removing the primary tumor and prior to a treatment following surgery or resection tumor and prior to a treatment following surgery or resection, (iii) selecting a colon or colorectal cancer subject for additional cancer treatment after treatment removing the primary tumor and prior to a treatment following surgery or resection, or (iv) determining, after treatment removing the primary tumor and prior to a treatment following surgery or resection, if the colon or colorectal cancer of a cancer subject is metastatic, comprising the steps of: a. measuring the level of methylated genomic DNA of SEPTIN9 (SEQ ID NO: 1) that is methylated in colon or colorectal cancer, but unmethylated in non-cancerous tissue, or a fragment thereof, in blood obtained from the subject prior to removing the primary tumor by surgery or resection (pre-treatment sample); b. measuring the level of methylated genomic DNA of SEPTIN9 (SEQ ID NO: 1) or a fragment thereof, in blood obtained from the subject after the primary tumor has been removed by surgery or resection and prior to a treatment following surgery or resection of the primary tumor (post-treatment sample), whereby an increased or equivalent amount of the methylated genomic DNA or fragment in the post-treatment sample after removal of the primary tumor compared to the pre-treatment sample indicates (i) a bad prognosis, (ii) the need for additional cancer treatment for the subject, (iii) to select the cancer subject for additional cancer treatment, or (iv) that the cancer is metastatic. (i) determining the prognosis of a colon or colorectal cancer subject after surgery or resection removing the primary tumor and prior to a treatment following the surgery or resection of the primary tumor, (ii) determining the need for additional medical treatment for a colon or colorectal cancer subject after treatment removing the primary tumor and prior to a treatment following surgery or resection tumor and prior to a treatment following surgery or resection, (iii) selecting a colon or colorectal cancer subject for additional cancer treatment after treatment removing the primary tumor and prior to a treatment following surgery or resection, or (iv) determining, after treatment removing the primary tumor and prior to a treatment following surgery or resection, if the colon or colorectal cancer of a cancer subject is metastatic, comprising the steps of: a. measuring the level of methylated genomic DNA of SEPTIN9 (SEQ ID NO: 1) that is methylated in colon or colorectal cancer, but unmethylated in non-cancerous tissue, or a fragment thereof, in blood obtained from the subject prior to removing the primary tumor by surgery or resection (pre-treatment sample); b. measuring the level of methylated genomic DNA of SEPTIN9 (SEQ ID NO: 1) or a fragment thereof, in blood obtained from the subject after the primary tumor has been removed by surgery or resection and prior to a treatment following surgery or resection of the primary tumor (post-treatment sample), whereby an increased or equivalent amount of the methylated genomic DNA or fragment in the post-treatment sample after removal of the primary tumor compared to the pre-treatment sample indicates (i) a bad prognosis, (ii) the need for additional cancer treatment for the subject, (iii) to select the cancer subject for additional cancer treatment, or (iv) that the cancer is metastatic. a. measuring the level of methylated genomic DNA of SEPTIN9 (SEQ ID NO: 1) that is methylated in colon or colorectal cancer, but unmethylated in non-cancerous tissue, or a fragment thereof, in blood obtained from the subject prior to removing the primary tumor by surgery or resection (pre-treatment sample); b. measuring the level of methylated genomic DNA of SEPTIN9 (SEQ ID NO: 1) or a fragment thereof, in blood obtained from the subject after the primary tumor has been removed by surgery or resection and prior to a treatment following surgery or resection of the primary tumor (post-treatment sample), whereby an increased or equivalent amount of the methylated genomic DNA or fragment in the post-treatment sample after removal of the primary tumor compared to the pre-treatment sample indicates (i) a bad prognosis, (ii) the need for additional cancer treatment for the subject, (iii) to select the cancer subject for additional cancer treatment, or (iv) that the cancer is metastatic. (i) a bad prognosis, (ii) the need for additional cancer treatment for the subject, (iii) to select the cancer subject for additional cancer treatment, or (iv) that the cancer is metastatic.