PURPOSE: To compare retrospectively the outcome of localized prostate cancers treated by curative external radiotherapy in which the negative lymphatic status was either surgically or radiologically assessed. METHODS AND MATERIALS: From January 1986 to December 1995, 112 patients with localized prostate cancers were found to have no evidence of lymphatic disease in the pelvis. N0 status was assessed either surgically (61 patients, group pN-) or after a CT scan procedure (51 patients, group cN0). The treatment consisted of conventional external radiotherapy using a four-fields box technique to a total dose of 65 Gy. The pelvis was never irradiated. RESULTS: The two groups did not statistically differ according to age, PSA level, Gleason score, T stage and hormonal therapy. Actuarial NED survival rates were 80% and 60% at five and ten years respectively. At ten years, the actuarial NED survival rates were 78% and 34% in the pN- and cN0 groups respectively (p = 0.003). The multivariate analysis corroborated the positive impact of lymphatic dissection before radiotherapy on disease free survival of T1-T2 patients, but not for T3 stages. CONCLUSIONS: This retrospective study suggests the inability of CT scan to accurately evaluate the lymph node status in carcinoma of the prostate. Systematic ilio-obturator nodal dissection is strongly recommended in early stages before curative radiotherapy. Only pN-patients should be included in high dose conformal irradiation trials.