The effect of interpregnancy interval on the birthweight of the subsequent child was investigated in a cohort of 5,938 women who registered for two consecutive pregnancies in the Collaborative Perinatal Project. Mean birthweight increased from 3,101 grams for intervals of less than 3 months to 3,193 grams for intervals of 15-17.9 months and remained stable thereafter (p for trend = 0.006). However, women with shorter intervals were younger, lighter weight, and less educated at the beginning of the first pregnancy than were women with longer intervals; the birthweight of their previous child was lower, and they were of marginally lower socioeconomic status. Adjustment for confounders reduced the maximum difference in mean birthweight by interval length from 92 to 39 grams, and blunted the trend for lower birthweights with shorter intervals (p = 0.45). Similarly, adjustment reduced the increased risk of low birthweight among women with the shortest intervals from 52 per cent to 12 per cent. We conclude that a short interpregnancy interval is primarily a marker for a woman who is otherwise at high risk, and that modification of this interval alone may be unlikely to have a major impact on low birthweight.