After Romsey's Conservative MP Michael Colvin died in a fire at his home in Tangley on 24 February 2000, Gidley was selected as the Liberal Democrat candidate for the resulting by-election. She won the by-election, on 4 May, with a majority of 3,311 votes and won in the 2001 and 2005 general elections. However, in 2005 her majority was reduced to only 125 votes, the smallest of any Lib Dem MP, by the Conservative candidate, Caroline Nokes. She was appointed as the party's spokeswoman on women's issues and older people from 2001. In January 2006, as Kennedy faced allegations of a drink problem, Gidley was one of 11 members of the front-bench team to write to Kennedy asking him to resign. Gidley chaired her party's Gender Balance Task Force, an initiative to get more women into politics.