The growth in length and weight of 91 poor urban Bangladeshi infants was monitored at monthly intervals from birth to 1 year. At birth 18%, 22% and 8% were below -2.00 standard deviations of the National Center for Health Statistics (NCHS) height-for-age, weight-for-age and weight-for-height, respectively and at 1 year the percentages were 40%, 40% and 7%. Infant weights over the first 6 months associated positively with both mother's BMI and percentage body fat. Higher mean infant weights from 6 to 12 months were associated with both higher educational status of the mother and monthly family income and the latter two variables provided the best model for predicting weight velocity from birth to 12 months. Internal Z-scores, corrected for day of measurement, provided clear evidence of catch-up/catch-down over the first 6 months with heavier and longer babies at birth showing catch-down while lighter and shorter babies demonstrated catch-up. Infants' weights were almost three times more variable than lengths. Monthly incremental variability of both infant weight and length increased sharply from 6 to 12 months with weight showing significantly more variability. The correlations of birth weight and length with subsequent distances and monthly increment revealed that the first 6 months were dominated by catch-up and catch-down but during the latter half of the year the growth phenomena were influenced mainly by earlier intra-uterine or genetic effects.