Nearly one-third of the Peruvian population burns biofuels for cooking. Similarly, approximately one-third of this population lives at high altitudes. Cooking with biofuels and living at high altitudes have been implicated in adverse perinatal outcomes. This study attempted to determine the risk of low birth weight (LBW) and/or preterm delivery in relation to biofuel use in inhabitants at high altitudes in Peru. A matchedcase study was performed in two high-altitude cities. All subjects (n = 190) were identified from public-hospital records. Cases were matched by hospital by week of birth and area of residence. Cases were defined as: women with an at-term but LBW newborn; or women with a preterm birth. Adjusted conditional logistic regression analyses were used to determine odds ratio. Fifty-two percent of the cases used biofuel whereas only 30% of the controls used biofuel. The use of biofuel for cooking was found to be strongly associated with increased risks of LBW, but was not significantly linked to preterm births.