Although a principal constituent of human skin, cutaneous water is difficult to study, and little is known about water behaviour in physiological and pathological conditions of the skin. It has been proposed recently that changes in dermal echogenicity measured by high-frequency ultrasonography reflect changes in dermal water content. To validate skin ultrasonography for assessment of dermal water changes we have studied the relationship between dermal echogenicity and skin water content determined by nuclear magnetic resonance technique. Twenty MHz ultrasound scanning of the dorsal and ventral forearm skin was performed in 59 healthy volunteers (age 18-65) and dermal echogenicity was determined by counting low echogenic pixels (LEPs) in skin images. 1H magnetic resonance spectra were obtained from the same regions and the ratio of areas under the water- and fat-specific peaks (W/F) were calculated to measure a relative water content. Acute dermal oedema (histamine weal) was studied in the same way in 40 individuals. Baseline dermal echogenicity correlated significantly with W/F, both in the ventral (r = 0.47) and dorsal (r = 0.57) forearm. Intradermal application of histamine caused a development of intradermal oedema which could be visualized by nuclear magnetic resonance imaging. In a corresponding ultrasound image oedema was seen as a low-echogenic area. The proportional increases in LEPs and W/F after histamine application were correlated, but the elevation in LEPs was 25-48% (95% confidence intervals) higher than that for W/F. These results suggest that high-frequency ultrasonography is a sensitive method for assessment of changes in dermal hydration. This technique may find important applications in comparative and non-invasive evaluations of dermal water in physiological conditions and in skin pathologies associated with oedema formation.