Interaction between cancer and stromal cells is critical for tumour cell growth and invasion. It has recently been suggested that mature adipocytes, a specific type of stromal cell that is abundant in the breast, affect the biological behaviour of some epithelial cell types. However, the nature of the interaction between mature adipocytes and breast cancer cells remains unclear. The present study has examined the effects of mature rat adipocytes, as the main cell type in adipose tissue, on the growth, apoptosis, and cell adhesion of breast carcinoma cell lines [human oestrogen receptor (ER)-positive cell lines MCF-7, ZR75-1, and T47-D; and the ER-negative mouse cell line MMT 060562] by analysing bromodeoxyuridine (BrdU) uptake, apoptotic indices, and immunohistochemical expression of E-cadherin. As a reference, the effects of rat preadipocytes (immature adipocytes) on the behaviour of the cell lines were examined. The cell lines were cultured in a three-dimensional collagen gel matrix with mature adipocytes or preadipocytes. Mature adipocytes significantly promoted BrdU uptake by all cell lines other than MMT 060562 cells. Preadipocytes decreased the uptake of T47-D cells and had no effect on that of MCF-7, ZR75-1 or MMT 060562 cells. Mature adipocytes or preadipocytes did not affect the apoptotic indices of any of the cell lines. Mature adipocytes did not influence E-cadherin expression in any of the cell lines, but preadipocytes clearly promoted E-cadherin expression by MCF-7 and T47-D cells, but not by R75-1 and MMT 060562 cells. These data suggest that mature adipocytes may be involved in the mechanisms regulating the growth of breast cancer mass through their growth-promoting effect on ER-positive tumour cells. Copyright © 2003 John Wiley & Sons, Ltd.