Introduction: Adequate nutrition is an indispensable factor for promoting growth and development in the first year of life. The replacement of human milk by cow milk, whenever needed, should be done carefully. Besides not supplying infant’s complete requirements, cow milk can trigger the advent of clinical situations demanding the partial or even total suspension of this food. Amongst these situations, lactose intolerance is one of the most common. Definition: Lactose intolerance is a clinical syndrome manifested by gastrointestinal symptoms triggered after its ingestion. Physiopathology and clinical manifestations: Lactase deficiency reduces the capacity of lactose’s digestion and absorption, leading to cramps, abdominal distension, flatulence and diarrhea, as a consequence of its osmotic effect on the colon. Classification: Lactase ontogenetic deficiency (adult primary hypolactasia), lactase secondary deficiency (consequent to small bowel mucosa injury), neonatal lactase deficiency (premature infants, born with less than 34 weeks of gestational age), congenital lactase deficiency (very rare genetic condition). Diagnosis: Essentially clinical, can be confirmed by therapeutic test, analysis of fecal pH and fecal reducing substances, lactose tolerance test, test of exhaled hydrogen, intestinal biopsy (enzymatic activity research), and genotype characterization. Treatment: Diet with partial or total restriction of lactose ingestion, indicated after confirmed diagnosis; use of an appropriate substitute for cow milk, with vitamin and mineral supplementation. Enzymatic replacement can be used as an alternative treatment for patients with partial intolerance.