He said he was happy to accept the challenge but as he had pneumonia last year, "it would not be wise" to have a bucket of cold water dumped on his head. His children, Robert, Lucy and Tim stepped up to take the challenge for their father.
Hawking encouraged others to donate to the Motor Neuron Disease Association (MNDA) to fund research into the different kinds of motor neuron disease, including amyotrophic lateral sclerosis.
Hawking was diagnosed with motor neuron disease in 1963 and was given two years to live. He is now 72 years old.