Ketogenic diets provide a non-pharmaceutical alternative for treatment of refractory epilepsy. When successful in reducing or eliminating seizures, medication numbers or doses may be reduced. Unexpected loss of ketosis is a common problem in management of patients on ketogenic diets and, especially when the diet is an effective treatment, loss of ketosis may be associated with an exacerbation in seizures. Identification of the cause of loss of ketosis is critical to allow rapid resumption of seizure control, and prevention of unnecessarily increased diet restriction or increased medication doses. Here an unusual environmental cause of loss of ketosis is described (contamination with starch-containing drywall dust), illustrating the extent of investigation sometimes necessary to understand the clinical scenario.