Dennis, along with Sherpa Tashi completed the first ascent of Nuptse on 16 May 1961. Other (British) members of the expedition were Joe Walmsley (expedition leader), Les Brown, Chris Bonnington Jim Swallow, Simon Clark and John Streetley. They travelled to Nepal in two Standard Vanguard estate cars by road, through countries including Turkey, Iran, Afghanistan and India. The climb was completed without the aid of oxygen.