[Congenital methaemoglobinaemia--a rare cause of dyspnoea and cyanosis].

Dyspnoea in young people often leads to the diagnosis of asthma. A young female (with related parents) showed symptoms of cyanosis, dyspnoea and fatigue during physical activity despite asthma medication. High levels of methaemoglobin were measured. Genetic testing showed homozygote type 1b5r-deficiency. Cyanosis and lacking effect of asthma treatment should lead to further diagnostic evaluation with arterial blood gas analyses, including assessment of methaemoglobin. Congenitally inherited methaemoglobinaemia is a rare disease, but its diagnosis is important to ensure correct handling and treatment.