We report the case of a 13-year-old boy with Costello syndrome and chronic constipation who received phosphate-containing Fleet-pediatric enemas at regular intervals. The day before admission he was given four enemas for severe constipation. Within 24 h the boy had developed severe hyperphosphatemia (phosphate 17.75 mmol/L), hypertonic dehydration (sodium 171 mmol/L) and severe hypocalcemia (calcium 0.56 mmol/L). With early intervention and treatment, the child survived without sequelae. Osmotically acting hypertonic phosphate enemas may result in severe toxicity. Children with developmental delay often display bowel dysfunction and chronic constipation and are therefore high-risk patients. The accepted opinion-that these enemas are not absorbed and therefore systemically inactive-is not true. Physicians should be aware of the potentially lethal complications of this treatment, which is part of everyday clinical practice.