Female vertebrates frequently ingest undeveloped ova and stillborn offspring. Two hypotheses have been proposed to interpret this behaviour: (1) it is a form of parental care, and (2) it recycles otherwise wasted energy, facilitating maternal recovery. Our study of Mexican lance-headed rattlesnakes provides the first quantitative description of cannibalism by postparturient rattlesnakes. We collected gravid females during June–July of 2004–2007 in central México, recording 239 litters from 190 females. Production of nonviable offspring was common (48% of litters). Most females (68%) ate some or all nonviable offspring, consuming an average of 11% of postpartum mass. Females consumed undeveloped ova and stillborn neonates in similar proportions. To evaluate factors that influenced the decision to cannibalize we used logistic regression. The best model included four predictive variables: (1) parturition date; (2) proportion of nonviable offspring mass per litter; (3) maternal investment index; and (4) snout–vent length (SVL). All variables exerted a positive effect on maternal cannibalism, although SVL was only a marginally significant predictor. Consumption of nonviable offspring confers energetic benefits to emaciated postparturient females. Energetic benefits are greatest when a large proportion of a litter is nonviable, while the pressure to cannibalize is greatest when late parturition shortens the time available to forage before the next reproductive event. Our results demonstrate the importance of maternal cannibalism in rattlesnakes, and suggest stronger support for the maternal recovery hypothesis than for the parental care hypothesis.