The gastrocnemius muscle flap is often insufficient in volume and arc of rotation for coverage of a large soft tissue defect of the knee and the upper third of the leg. Therefore we developed a new concept of the flap which combines soleus and gastrocnemius muscles, named the 'gastrocnemius with soleus bi-muscle flap'. In 16 cadavers we studied the location and number of perforators, which penetrate the gastrocnemius muscle through the soleus muscle. In all cases perforators were found in the distal half of the gastrocnemius muscle. Angiography in one fresh cadaver confirmed that the soleus muscle could receive the reversed flow from the gastrocnemius muscle perforators. We subsequently treated a patient with exposed proximal tibia with this flap. This flap is useful to cover a large soft tissue defect of the knee and the upper third of the leg.