In undiagnosed and untreated children, the accumulation of precursor metabolites due to the deficient activity of galactose 1-phosphate uridylyltransferase (GALT) can lead to feeding problems, failure to thrive, liver damage, bleeding, and infections.

Without intervention in the form of galactose restriction, infants can develop hyperammonemia and sepsis, possibly leading to shock.

[2] Duarte galactosemia is caused by mutations that produce an unstable form of the GALT enzyme, with reduced promoter expression.

[2] In most regions, galactosemia is diagnosed as a result of newborn screening, most commonly by determining the concentration of galactose in a dried blood spot.

While awaiting confirmatory testing for classic galactosemia, the infant is typically fed a soy-based formula, as human and cow milk contains galactose as a component of lactose.

[citation needed] There is no cure for GALT deficiency, in the most severely affected patients, treatment involves a galactose free diet for life.

Long-term manifestations of the disease including ovarian failure in females, ataxia, and growth delays are not fully understood.

[2] Routine monitoring of patients with GALT deficiency includes determining metabolite levels (galactose 1-phosphate in red blood cells and galactitol in urine) to measure the effectiveness of and adherence to dietary therapy, ophthalmologic examination for the detection of cataracts and assessment of speech, with the possibility of speech therapy if developmental verbal dyspraxia is evident.