"Billion Dollar Brain" is the third of the Harry Palmer film series, preceded by "The Ipcress File" (1965) and "Funeral in Berlin" (1966). It is the only film in which Ken Russell worked as a mainstream 'director-for-hire', and the last film of Françoise Dorléac. A fourth film in the series, an adaptation of "Horse Under Water", also to be released by United Artists, was tentatively planned but never made. Caine played Palmer in two later films, "Bullet to Beijing" and "Midnight in Saint Petersburg".