114 women in fully nursing, were studied, according to the length of lactational amenorrhea and their relation with general characteristics and ethnic group. Of the 114 mothers, 66 (58%) recovery their menses before the 6th postpartum month. The length of lactation was 100.7 +/- 5.2 and 277.9 +/- 10.5 days, for those mothers who were with menses and amenorrhea until the 6th postpartum month, respectively. A group of Mapuche women had a higher length of lactational amenorrhea and we found differences in the frequency of suckling episodes at night. This study corroborate, that short length of lactational amenorrhea is a condition highly frequent in Chilean population and it does not have a relation the nursing pattern in general population. However, the difference in the length of lactational amenorrhea in specific ethnic group, can include some general or behavioral factors of breast-feeding.