To investigate whether sodium supplementation increases weight gain in babies with grade 3 Pierre Robin sequence if their urinary sodium is low. Retrospective review of all medical, dietitian, and nursing notes of babies admitted from 2000 to 2007, inclusive. Tertiary center for cleft care. A total of 33 babies requiring nasogastric and nasopharangeal intubation for Pierre Robin sequence were identified. Those for whom weights could not be measured accurately (n = 4) were excluded from further study. Urinary sodium was low and weight gain poor in 21 of the remaining 29 babies (72%), and oral sodium supplementation was given to establish and maintain a normal urinary sodium (10 to 20 mmol/L). Mean daily weight gain. The mean daily weight gain of the 18 babies with low urinary sodium admitted in the first few days of life improved from 20 to 34 g (p < .001) following oral sodium supplementation. This gave a mean difference of 14 g (lower quartile = 6 g, upper quartile = 25 g, 95% confidence interval = 20, 90% confidence interval = 10). The mean daily weight gain of the eight patients who did not require sodium supplementation was 33.6 g. Urinary sodium tests are simple and noninvasive and allow monitoring of urinary sodium. If low, adequate supplementation improved weight gain (p < .001), reversing failure to thrive in babies with severe Pierre Robin sequence. These findings have been unreported previously.