The duration and determinants of interbirth intervals among women of reproductive age in Karak, Jordan were examined in October 2003. A multistage sampling technique was used to select 1109 ever-married women aged 15-49 years who contributed to 4349 interbirth intervals. Data were obtained by interview questionnaire and analysed with life table, Kaplan-Meier survival and Cox regression analyses. The median interbirth interval was 27.40 months. Longer interbirth interval was independently predicted by breastfeeding > or = 12 months, modern contraceptive use and pregnancy wastage; by more surviving children, presence of boys only or both boys and girls at the interval onset; by woman's higher education, older age and longer marriage; and by ideal spacing conforming with family planning norms. Concerted efforts to encourage modern contraceptive use, extend breastfeeding, promote small family size, address gender preferences and reinforce the minimum age at marriage should be made.