Recent research has shown that children born before and after short birth intervals run a considerably greater risk of dying in infancy or childhood than do others. This report investigates which women have short interbirth intervals, under what circumstances, and for what reasons. The analysis uses data from the Malaysian Family Life Survey to examine influences on the two main behaviors--breastfeeding and contraceptive use--that affect interval length, and assesses the impact of these same variables on the probability of having a birth interval of less than 15 months. The analysis shows that many of the independent variables affect breastfeeding and contraceptive use in opposite directions, with no significant net effect on the likelihood of a short interval. For example, a woman's education is negatively related to the probability that she breastfeeds, positively related to the probability that she uses contraceptives, and has no significant effect on the likelihood that the interpregnancy interval is less than 15 months. Having a family planning clinic nearby is associated with less breastfeeding, offsetting whatever positive effects family planning clinics have on contraceptive use in terms of the percentage of birth intervals that are so short as to be detrimental to infant and child health. Hence, factors that increase contraceptive use do not necessarily reduce the incidence of short interbirth intervals, because they are also associated with reduced breastfeeding. The authors simulate the proportion of intervals that would be short for alternative combinations of breastfeeding and contraceptive use in the population and show that over the period covered by the data (1961-1975), breastfeeding had a considerably greater effect on preventing short interbirth intervals than did contraceptive use.