We reach the conclusion that histidinaemia in the typical form (autosomal recessive impairment of L-histidine ammonia lyase activity (EC 4.3.1.3)) is not a ‘disease’ in man. Retrospective and prospective studies (Rosenmannet al., 1983; Coulombeet al., 1983) together indicate that the prevalence of disadaptive phenotypes (e.g. impaired intellectual or speech development, seizures, behavioural or learning disorder) in the histidinaemia population, is not higher than the frequency of these functional disorders in the non-histidinaemia population. However, one cannot exclude the possibility that histidinaemia is a risk factor for development of an unfavourable CNS phenotype, in particular individuals under specific circumstances (e.g. abnormal perinatal events). From this viewpoint, we propose that newborn screening for early diagnosis and treatment of histidinaemia does not meet the criteria for a public health service. Screening for continuing research on histidinaemia remains justifiable, for example, to discern whether there are atypical forms of the biochemical and enzymatic phenotypes.