PPARgamma and C/EBPalpha are critical transcription factors in adipogenesis, but the precise role of these proteins has been difficult to ascertain because they positively regulate each other's expression. Questions remain about whether these factors operate independently in separate, parallel pathways of differentiation, or whether a single pathway exists. PPARgamma can promote adipogenesis in C/EBPalpha-deficient cells, but the converse has not been tested. We have created an immortalized line of fibroblasts lacking PPARgamma, which we use to show that C/EBPalpha has no ability to promote adipogenesis in the absence of PPARgamma. These results indicate that C/EBPalpha and PPARgamma participate in a single pathway of fat cell development with PPARgamma being the proximal effector of adipogenesis.