Background: Studies done in Santiago, Chile show that menses return before the sixth month of puerperium in 50% of lactating women, even in those that continue with exclusive breast feeding. Aim: To study the length of lactational amenorrhea in a group of women living in Southern Chile. Material and methods: One hundred fourteen women giving exclusive breast feeding, were followed from the third postpartum month, to determine the length of lactational amenorrhea. Its relationship with general characteristics of the mothers and children and breast feeding pattern was also studied. Results: Sixty six women (58%) recovered their menses before the 6th postpartum month and their length of lactational amenorrhea was 101 ± 5 days. In the rest of the sample, the length was 277 ± 10 days. No differences in characteristics of the mothers and children or breast feeding pattern, were observed between these two groups. Conclusions: A short lactational amenorrhea is common in the Chilean population. Clinical characteristics or pattern of breast feeding do not explain the length of LA in this population ( Rev Méd Chile 2002; 130: 66-70)