Although primary cilia affect the activity of the Hedgehog (HH) pathway, the response to cilia loss varies among tumour types. Ho et al. show that chondrosarcomas (cartilage tumours arising from chondrocytes) have fewer cilia than normal chondrocytes. Transgenic mice with partial cilia loss developed similar cartilage tumours to mice with chondrocyte-specific activation of HH; double-transgenic mice developed more tumours and had further activation of the HH pathway. Disruption of cilia in human chondrosarcoma explants also activated HH signalling, leading to increased proliferation and reduced apoptosis. Therefore, cilia loss may be sufficient to induce these tumours.