Mannosidosis remains an extremely rare entity (9 published cases). Our personal experience, based upon 5 cases, suggests that the diagnosis may be based upon both positive as well as negative clinical and paraclinical data, which differentiate the disorder from other diseases which it resembles. The use of simple methods--thin layer chromatography of oligosaccharides and measurement of serum mannosidase at pH 3.5 and 4.5--render diagnosis easy. It should, however, be mentioned that there exists no valid treatment for this apparently autosomal recessive disorder but that ante-natal detection is theoretically possible.