This California Law Review Comment argues that by applying a proportionality analysis to Dillon's first-degree felony-murder sentence the court opened the much criticized felony-murder rule to constitutional attack in most of its applications. Whenever the rule is needed to perform its intended function -- the deterrence of accidental and negligent felony killing -- its application results in unconstitutional penalties. The background and facts of the Dillon case and the plurality opinion are summarized in Part 1. Part 2 examines the preexisting law of felony murder and prior applications of the constitutional prohibition against cruel or unusual punishment. Part 3 argues that the felony-murder rule, which imposes penalties without regard to culpability, is inconsistent with a constitutional requirement that penalties be proportionate to culpability. Consequently, courts can refuse to apply the felony murder rule in many cases on the ground that the penalty is disproportionate to the defendant's conduct. It is concluded that the supreme court should declare the felony-murder rule void because its inherent disproportionality renders it incapable of constitutionally performing its intended function. One hundred and forty eight reference notes are provided. (Author abstract modified)