By means of an immunocytochemical method the 1.25-dihydroxyvitamin D [1.25(OH)2D] receptor status of tumours from 136 patients with primary carcinoma of the breast was determined. Patients with receptor-positive tumours had significantly longer disease-free survival than those with receptor-negative tumours (Chi2 = 4.01, p less than 0.05). 1.25(OH)2D3 inhibits the proliferation of several established human breast cancer cell lines in vitro. Effects of 1.25(OH)2D3 on breast tumour growth in vitro were assessed by means of the nitrosomethylurea-induced rat mammary tumour model of hormone-responsive breast cancer. Treatment of tumour-bearing animals with 0.1 microgram of the synthetic analogue, 1 alpha-hydroxyvitamin D3, three times weekly produced significant inhibition of tumour progression. Taken together, these studies suggest that the levels of 1.25(OH)2D occurring in vivo may exert an inhibitory effect on receptor-positive tumours. Further studies are required to evaluate the role of vitamin D metabolites in the treatment of human malignant disease.