Non-invasive methods for monitoring the effect of an agent for treating colorectal cancer, comprising (1) collecting fecal samples from healthy subjects, subjects having precancerous lesions, and subject having precancerous lesions and being treated with an anti-cancer agent; (2) isolating total microbial genomic DNA from the fecal samples to provide total microbial genomic DNA; (3) comparing the total microbial genomic DNA using fingerprint spectrum analysis; (4) identifying key fingerprint bands correlated with the effect of the anti-cancer agent; (5) identifying key microorganisms associated with the key fingerprint bands; (6) designing microbial sequence-specific primers and probes; and (7) determining the quantitative differences of the key microorganisms in fecal samples to identify an indicator microorganism for monitoring the effect of the anti-cancer agent.