This research investigates in detail the surprisingly successful US box-office performance of Jess Franco’s first women-in-prison (WIP) production, 99 Women (1968), which for one week in May 1969 had topped the Variety Top-50 box office chart. It documents the unusual production history of this Franco film, the role of UK producer Harry Alan Towers in enabling Franco’s pipeline of productions at the end of the 1960s, 99 Women’s international distribution, and the various reasons that have been hypothesized for its box-office success. It then briefly examines the box-office performances of two other proximate Franco productions—Succubus (1967) and Venus in Furs (1968)—and outlines the contributions made by Vienna-born actress Maria Rohm to the success of 99 Women and various other Franco-Towers films that had featured her in major roles such as Eugenie … The Story of Her Journey into Perversion (1969).