OBJECTIVE: To investigate the independent and combined associations between female and male body mass index (BMI) on the probability of achieving a live birth after treatments with in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) under adjustment for relevant covariates. DESIGN: Population-based cohort study. SETTING: Danish national registers. PATIENT(S): Patients with permanent residence in Denmark receiving IVF or ICSI treatment with use of autologous oocytes from January 1, 2006, to September 30, 2010. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): Live birth. Analyses were adjusted for age and smoking at treatment initiation and results stratified by BMI groups and presented by IVF/ICSI treatment. RESULT(S): In total, 12,566 women and their partners went through 25,191 IVF/ICSI cycles with 23.7% ending in a live birth. Overweight and obese women with regular ovulation had reduced odds of live birth (adjusted OR 0.88, 95% CI 0.79-0.99 and adjusted OR 0.75, 95% CI 0.63-0.90, respectively) compared with normal-weight women. IVF-treated couples with both partners having BMI =25 kg/m(2) had the lowest odds of live birth (adjusted OR 0.73, 95% CI 0.48-1.11) compared with couples with BMI