The authors evaluated the US patterns of 50 prostatic carcinomas before and after radiation therapy, analyzing the correlation of US patterns with histologic (bioptic) findings and with treatment outcome. Before radiation therapy, 28 tumors were hypoechoic, 8 were isoechoic and 14 mixed. In 18 of 28 hypoechoic carcinomas, the lesion showed a progressive increase in echogenicity, becoming completely isoechoic 9-18 months after the end of treatment. In this group of patients the final response to treatment was complete in 17 cases and partial in 1; during the follow-up (24-90 months, mean 48 months) neither local recurrences nor distant metastases were observed. In 10 of 28 hypoechoic carcinomas, a hypoechoic area of variable size was still clearly recognizable within the lesion after a minimum period of 18 months since the end of treatment. Nine of these patients underwent histology and persistent carcinomas were found in 8 of them; one patient developed distant metastases. In the group of isoechoic and mixed carcinomas, no significant differences were observed in the US patterns relative to treatment outcome. Persistent prostatic carcinoma after radiation therapy seems not to affect its US pattern. If the tumor is hypoechoic before treatment, the persistence of a hypoechoic area within the lesion 18 months after the end of treatment must be regarded as a possible therapeutic failure and histologically verified. On the contrary, the evolution of the lesion toward isoechogenicity is usually related to a favorable outcome. If the tumor is isoechoic or mixed before treatment, a reliable US evaluation is not possible and the correct assessment of the response to treatment can only be made with multiple biopsies.