Background/purpose: Treatment of atopic dermatitis (AD) and psoriasis requires their differentiation from other eczematoid dermatitis and a determination of disease severity. However, both can be clinically difficult and the findings subjectively interpreted. We investigated the utility of in vivo autofluorescence (AF) measurements for diagnosis of both diseases, and determination of severity. Materials and methods: Thirty patients with AD and 30 with psoriasis were recruited, together with sex- and age-matched patients with healthy skin. AF intensity was measured using the EcoSkin((R)) fluorescence video dermatoscope. In AD and psoriasis patients, AF in non-sun-exposed lesional and non-lesional skin was measured. To identify the locations that reflect characteristics of AD, AF was also measured at the other sites in the patients with AD. Results: AD was associated with lower AF and psoriasis with higher AF intensity peaking around 620 nm. In addition, skin AF intensity of each disease was associated with severity of lesion. Conclusions: Non-invasive measurement of skin AF in vivo can aid in diagnosis of AD and psoriasis as well as in treatment monitoring.