This paper proposes how adult pervasive developmental disorder（PDD）should be diagnosed. PDD patients diagnosed in adulthood for the first time differ from PDD children in the following points : first, they have comparatively inconspicuous PDD traits which have gone unnoticed for years, and second, the PDD features can be modified by comorbid psychiatric disorders. It has also been reported that PDD symptoms such as social interaction disturbance or communicative problems of verbal patients with autism would gradually improve with maturity. These factors make the clinical entity of adult PDD obscure. Therefore, it is difficult to diagnose adult PDD based on the categorical concept of disorder only from assessing present status without taking a precise developmental history. On the other hand, there is the concept of “Autism spectrum disorder（ASD）”. This concept of a spectrum could be adapted for adult cases including a wide range of PDD. To do so,however, two issues must be resolved : 1）whether it is possible to set a common diagnostic threshold, and 2）how the use of the term “spectrum”invites the misperception of PDD as only unidimensional. For their resolution, the author proposes two novel concepts : multidimensional trait assessment for PDD（or ASD）diagnosis and a working diagnosis of probable PDD（or ASD）. The former can be depicted as a radiating chart having multi-axes of PDD （or ASD）traits, whereas a spectrum assessment would be only a single point on a straight line. This trait assessment can provide tailor-made help for individual patients. The latter can allow proper treatment of underdiagnosed patients who have obvious PDD features but a poor developmental history. These concepts will enable us to treat elusive cases of adult PDD.