Introduction: Women with gestational diabetes mellitus (GDM) have an increased risk of developing type 2 diabetes postpartum. Breastfeeding has been associated with a decreased risk of various metabolic diseases. The aim of this study was to evaluate the impact of the duration of lactation on leptin levels in women with previous GDM compared with women with normal pregnancy. Materials and methods: We performed a secondary analysis of a database of a prospective comparative study in which we evaluated 43 women with GDM and 43 normotensive and euglycemic pregnant women at pregnancy and at postpartum. Participants were classified according to the lactation duration in short (< 6 weeks) or long duration (> 6 weeks - < 6 months) and the leptin levels were measured. Results: Compared with women with GDM who lactated < 6 weeks, women with longer duration of lactation showed greater weight loss at postpartum and lower leptin levels. This difference remained statistically significant after adjustment for weight. In the control group, women with longer duration of lactation had lower triglyceride and insulin levels, and insulin resistance than women with short duration. Conclusions: The longer duration of breastfeeding was associated with lower levels of leptin and better metabolic profile in the early postpartum period in women with previous GDM and normal pregnancy respectively. Key words: leptin, breastfeeding, gestational diabetes mellitus, insulin, insulin resistance