Although the disorder is present from birth, symptoms of Hartnup disease may manifest in infancy, childhood, or early adulthood. Symptoms may be precipitated by sunlight, fever, drugs, or other stresses.

Poor nutritional intake nearly always precedes appearance of symptoms. Symptoms and signs are due to niacinamide deficiency and resemble those of pellagra, particularly the rash on parts of the body exposed to the sun; mucous membrane and neurologic symptoms also occur. Neurologic manifestations include cerebellar ataxia and mental abnormalities. Intellectual disability, short stature, headache, and collapsing or fainting are common.

Diagnosis of Hartnup disease is made by showing the characteristic amino acid excretion pattern in the urine. Indoles and other tryptophan degradation products in the urine provide supplementary evidence of the disease.

Prognosis is good, and frequency of attacks usually diminishes with age. The number and severity of attacks can be reduced by maintaining good nutrition and supplementing the diet with niacin or niacinamide 50 to 100 mg po tid. Attacks may be treated with nicotinamide 20 mg po once/day.