Using data from 242 exclusively breastfeeding mother-infant pairs, newborn elimination patterns were analyzed. Sensitivity (Se) and Specificity (Sp) of day 4 (72-96 hours) wet and soiled output, in addition to the timing of onset of lactation, in identifying cases of breastfeeding inadequacy (defined as neonatal weight loss >or= 10% of birth weight) were examined. The usefulness of 2 measures in parallel was also explored. Median number of diapers (wet, soiled) on days 1, 4, and 7, respectively, was 2, 3; 5, 4; and 7, 6. The most efficient day 4 predictor of breastfeeding inadequacy was soiled diaper output <or= 3 (Se = .75, Sp = .66). Sensitivity improved when used in parallel with delayed onset of lactation (>or= 72 hours); Se = .86 (95% confidence interval, .73-.99) and Sp = .59 (.55-.63). Fewer than 4 soiled diapers on day 4 when used in conjunction with delayed onset of lactation may be indicative of breastfeeding inadequacy, but low specificity will result in many false positives.