Between 1979 and 1983, 127 patients with Stages B2 or C rectal cancer treated with surgery plus postoperative adjuvant radiotherapy (RT group) and 122 patients treated with surgery alone (S group) were compared to evaluate the effect of postoperative radiotherapy on survival and disease recurrence. Each group was stratified into subgroups according to stage and tumor differentiation as follows: Subgroups BW (Stage B2 and well-differentiated tumor), BM (Stage B2 and moderately differentiated tumor), CW (Stage C and well-differentiated tumor), CM (Stage C and moderately differentiated tumor), and P (poorly differentiated tumor). Ninety-five percent of the patients were followed until death or, if alive, to five years after surgery. Postoperative radiotherapy was associated with a reduced five-year survival rate in Subgroup BW (67 vs. 87 percent; P = 0.02). In the remaining subgroups of the RT group, there was a statistically insignificant trend toward a worse survival rate (56 vs. 65 percent, 47 vs. 64 percent, 41 vs. 46 percent, and 50 vs. 36 percent for Subgroups BM, CW, CM, and P, respectively). The local failure rates for the S group and RT group were 10 vs. 23 percent (P = 0.15) in Subgroup BW, 32 vs. 21 percent (P = 0.4) in Subgroup BM, 24 vs. 25 percent (P = 0.6) in Subgroup CW, and 18 vs. 18 percent (P = 0.6) in Subgroup CM, respectively. Eight percent (9/127) had severe or life-threatening radiation-related complications. Postoperative adjuvant radiotherapy alone did not improve the survival of patients with Stages B2 or C rectal cancers. It may have led to worsened survival in the subgroup of patients with well-differentiated Stage B2 rectal cancer.