A six-month old infant developed a breakdown of the milieu interieur (hypotonic dehydration and metabolic alkalosis) due to chronic electrolyte losses by perspiration potentiated by a hot summer. The hypoelectrolytaemic shock led to cerebral oedema associated with unconsciousness, spasms and protracted hypoxia which led to the development of irreversible posthypoxic encephalopathy and secondary epilepsy. Repeated inflammations of the airways and progressing cachexia were at first associated with the neurological affection of the patient. Not even frequent prolapses of the rectum, the presence of nasal polyps and cultivation findings of Pseudomonas aeruginosa associated with the relapsing „obstructional“ bronchitis aroused suspicion of cystic fibrosis (CF). The diagnosis was made only at the age of seven years by repeated positive sweat tests and the detection of two mutations of the CFTR gene. In the differential diagnosis of hypoelectrolytaemic disbalance it is necessary to take into consideration also CF, the initial stages of which need not always have a typical course. Early assessment of the diagnosis and initiation of the appropriate treatment may decide in a fundamental way on the patient’s further fate.

        Key words: hypotonic dehydration, metabolic alkalosis, cystic fibrosis