To evaluate peripheral infiltrative growth (inf) of breast catrcinomas as a prognostic factor, overall survival probabilities in 169 patients with tumor less than 5 cm in diameter were analysed in relation to the degree of inf (alpha, beta, gamma) using Kaplan-Meier method. There were statistically significant correlations between inf alpha and beta, inf alpha and gamma. On the other hand, the boundary echo (BE) observed at the peripheral margin of tumor in ultrasonogram is considered to reflect inf patterns of breast carcinomas. Therefore, BE was classified into three types as follows. Type I; negligible or regular thin marginal hyperecho. Type II; irregular thick marginal hyperecho. Type III; radiating boundary echo. These three types were matched to the patterns of infiltration so as to find any correlation between ultrasonogram and a corresponding histrogical specimen. Three types of ultrasonographic infiltrative grades (U-INF alpha, beta, gamma) were defined as follows. U-INF alpha; Type I BE is observed almost all surface of tumor. U-INF beta; Type II or III BE is observed in less than 50% of tumor surface. U-INF gamma; Type II or III BE is observed in over 50% of tumor surface. Total of 108 cases with tumor less than 3cm in diameter were received preoperative ultrasonographic studies. U-INF had highly significant correlation with inf and the occurrence of lymph node metastasis.