The aim of this study was to determine the chemosensitivity of infiltrating lobular breast carcinoma (ILC) in comparison with infiltrating ductal carcinoma (IDC). Between 1987 and 1995, 457 patients with invasive T2>3 cm-T4 breast carcinomas were treated with primary chemotherapy (CT), surgery, radiation therapy. Clinical response, the possibility of breast preservation, pathological response and survival were evaluated according to the histological type. In order to evaluate the biological differences between ILC and IDC patients and their implication with regard to tumour chemosensitivity, additional immunohistochemical stainings (oestrogen receptor (ER), Bcl2, p53, c-erbB-2 and Ki67) were performed on 129 pretherapeutical specimens. 38 (8.3%) ILC were diagnosed by core needle biopsy before CT. ILC was an independent predictor of a poor clinical response (P=0.02) and ineligibility for breast-conserving surgery after neoadjuvant chemotherapy (P=0.03). Histological and biological factors predicting a poor response to CT (histological grade, ER, Ki67 and p53 status) were more frequent in ILC than in IDC patients. After a median follow-up of 98 months (range: 3-166), the low chemosensitivity of ILC did not result in a survival disadvantage. Our results demonstrate that ILC achieved a lower response to CT than IDC because of their immunohistochemical profile. Preoperative CT did not allow a high rate of conservative treatment for ILC and therefore the use of neoadjuvant CT for ILC patients should be questioned.