Imaging devices for long-wavelength near-infrared light (e.g., MCT, InGaAs) and hyperspectral imaging devices using these wavelengths have recently been developed. This range of wavelengths is especially effective for measuring the composition of objects because it includes the absorption spectra of molecular vibrations. In this study, long-wavelength near-infrared spectral imaging was used to evaluate allergic dermatitis. Allergic dermatitis can be caused by 2 types of hypersensitivity, immediate (type I) and delayed (type IV), which are activated by different mechanisms. However, both types of dermatitis are characterized by erythema and localized tissue swelling of the affected area, and it is therefore difficult to determine the type of allergic response by macroscopic inspection. Near-infrared spectral imaging has attracted attention as a non-destructive inspection method, but while several studies have focused on the detection of edema by near-infrared spectroscopy, the previously reported methods have not attempted to differentiate between different types of allergic dermatitis by detecting intracutaneous targets specific to the different activation mechanisms. This study aimed to develop a method for the assessment of allergic dermatitis by using the long-wavelength near-infrared spectrum to detect intracutaneous allergic type-specific targets. Such a method was realized by establishing a spectral classifier for the spectra of type I and type IV allergic dermatitis reactions. A spectral classifier was established from the spectral datasets of histamine-induced cutaneous reaction (immediate type) and contact hypersensitivity erythema elicited by squaric acid dibutylester (SADBE; type IV), and a canonical discriminant analysis achieved very accurate detection of normal skin and types I and IV allergic dermatitis (normal skin: 87.2%, histamine induced reaction: 71.0%, SADBE contact hypersensitivity: 95.8%). The classifier was next applied to spectral images of 2 other skin conditions, red flare activated by methyl nicotinate (normal skin) and metal allergy (type IV), and these validation datasets were also correctly classified: the red flare induced by methyl nicotinate was categorized as normal skin and the contact hypersensitivity activated by the metal allergy patch test was categorized as a type IV allergic reaction. These results suggest a possible application of near-infrared spectral imaging to the assessment of allergic dermatitis.