The authors aimed to identify factors associated with exclusive breastfeeding (EBF) among 117 Peruvian mothers planning to breastfeed exclusively. Data were collected on days 0 and 3, and months 1, 3, and 6. Exclusive breastfeeding status was evaluated with a 24-hour recall of infant diet. Exclusive breastfeeding rates were 74%, 72%, and 35% at 1, 3, and 6 months, respectively. At 3 months, lower maternal education, greater breastfeeding frequency (day 3), greater breast pain (day 3), and depot medroxyprogesterone acetate use (3 months) were associated with EBF, after adjusting for EBF intentions. At 6 months, greater infant birth weight and mother-not employed were associated with EBF, after controlling for EBF intentions. More educated and working mothers, and infants with lower birth weight should be targeted in interventions to promote EBF in urban Peru. Research is also warranted to explore the factors linking depot medroxyprogesterone acetate use and breast pain with EBF duration.