5-(Hydroxymethyl)uracil may be formed during the course of the two processes: oxidation/hydroxylation of thymine with resultant formation of 5-hydroxymethyluracil paired with adenine (produced by reactive oxygen species) and reacting of reactive oxygen species with 5-methylcytosine forming 5-hydroxymethylcytosine, followed by its deamination to 5-hydroxymethyluracil mispaired with guanine. Determination and quantification of hydroxymethyl uracil in urine serve as a potential biomarker of oxidative DNA damage. It was discovered, that urinary 5-hydroxymethyluracil might reflect the oxidative stress/chronic inflammation in colorectal cancer although the diagnostic performance for early-detection is moderate.