OBJECTIVE: Extragonadal estrogen biosynthesis is relevant for the regulation of bone metabolism. The aims of this paper were: (i) to examine CYP19 (aromatase) gene expression in primary cultures of osteoblasts under several hormone and cytokine treatments and (ii) to study the promoter usage of CYP19 in these cells. METHODS: Primary cultures of osteoblasts were obtained from healthy donors. The effects of vitamin D and other factors on CYP19 expression were analysed by semiquantitative RT-PCR. Furthermore, CYP19 alternative promoter usage under the different treatments was characterized by RT-PCR. RESULTS: CYP19 transcripts were detected in cultured human osteoblasts in serum-free conditions. Vitamin D, dexamethasone, 17beta-estradiol and testosterone increased transcript levels of CYP19, whereas interleukin-1beta or tumor necrosis factor alpha decreased them. Aromatase mRNA produced under treatment with vitamin D was transcribed from promoters I.4 and I.3, while stimulation with dexamethasone or dexamethasone plus vitamin D also involved promoter I.2. Testosterone activated promoters I.2 and I.4. CONCLUSIONS: Our results suggested that vitamin D, testosterone, estrogens and glucocorticoids regulate CYP19 gene expression in human primary osteoblasts and the main promoter used appears to be promoter I.4. Promoters pII and I.3 seem to be related to basal transcription and may mediate estrogen stimulation, while promoter I.2 seems to play a role in the effect of glucocorticoids. These findings indicate that vitamin D and several hormones regulate local estrogen synthesis in human osteoblasts mainly through usage of promoters I.4 and I.3.