||Minimising the use of animals in experiments is universally recognised by scientists, governments and advocates as an ethical cornerstone of research. Yet, despite growing public opposition to animal experimentation, mounting evidence that animal studies often do not translate to humans, and the development of new research technologies, a number of countries have reported increased animal use in recent years. In the USA—one of the world's largest users of animals in experiments—a lack of published data on the species most commonly used in laboratories (eg, mice, rats and fish) has prevented such assessments. The current study aimed to fill this gap by analysing the use of all vertebrate animals by the top institutional recipients of National Institutes of Health research funds over a 15-year period. These data show a statistically significant 72.7% increase in the use of animals at these US facilities during this time period—driven primarily by increases in the use of mice. Our results highlight a need for greater efforts to reduce animal use. We discuss technical, institutional, sociological and psychological explanations for this trend.