Preoperative management of advanced rectal cancer often includes chemoradiotherapy, but little is known about the late complications of radiotherapy. However, these are usually serious, making determination of the characteristics of late complications after radiation therapy critical. Accordingly, we investigated the complications occurring after adjuvant pelvic radiation therapy in patients with advanced rectal cancer. We enrolled 34 consecutive patients with TNM stage III rectal cancer who had undergone curative surgery with adjuvant pelvic radiation therapy. Data on the type of complication/organ involved, the number of complications, the phase of onset, and the treatments used were reviewed. Patients who experienced gut complications or edema were less likely to have their complications resolved than those with complications due to infection. Similarly, patients with multiple complications and late-onset complications were also less likely to have their complications resolved than those with single complications and those with early-onset complications, respectively. Adjuvant radiation therapy in patients with resected advanced rectal cancer was associated with various complications, characterized by late onset and impaired resolution. Therefore, patients indicated for radiation therapy should be selected with great caution.