Abstract: This article examines the interwar expansion of the ACLU agenda to incorporate nonpolitical speech. In the early 1920s, it was ACLU policy to contest an obscenity regulation only if its underlying motivation was the suppression of disfavored political or economic views. By 1931, however, the ACLU was an aggressive advocate of artistic freedom and birth control and the undisputed leader of the anti-censorship campaign. The catalyst for change was a postal censorship dispute involving a sex education pamphlet, The Sex Side of Life: An Explanation for Young People, written by the former suffragist and outspoken birth control activist Mary Ware Dennett. Postal authorities declared the pamphlet obscene despite its many endorsements from medical practitioners, religious groups, and government agencies. ACLU board members agreed to defend The Sex Side of Life because they believed that liberalizing access to scientific knowledge promoted the public interest in a familiar progressive fashion and would highlight the dangers of suppressing subversive ideas. Unexpectedly, however, the litigation precipitated a far more sweeping anti-censorship campaign. Dennett’s heavily publicized conviction, overturned by the Second Circuit on appeal, generated popular hostility toward obscenity laws and convinced ACLU attorneys that speech should be protected regardless of its social value. With that shift, the ACLU inched closer toward a new model of civil liberties premised on individual expressive freedom.