Galactosemia is a rare hereditary disorder in which the onset and severity of the first clinical signs can vary significantly, from the first days of life to several months. The pathogenesis of galactosemia is based on a deficiency of the enzyme involved in the conversion of galactose to glucose. The accumulation of galactose and its toxic metabolites leads to the development of the corresponding clinical symptom complex. Neonatal screening is currently performed in the Republic of Belarus to ensure early diagnosis of galactosemia and prevent life-threatening complications. However, the disease can progress rapidly, requiring a multidisciplinary approach to diagnosis and treatment. Physicians must remain vigilant while awaiting screening results. The primary treatment method is a completely galactose-free diet. With timely initiation of dietary therapy, the prognosis for life and health is favorable. The article presents modern literature data and provides a clinical observation of two cases of classical galactosemia type I associated with «severe» mutations of the GALT gene, such as deletion Q188R, K285N, which manifested and progressed, however, completely differently, which, of course, is of academic interest to a practicing physician. 

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