OBJECTIVE In dermatology high resolution ultrasonic systems proved to be valuable in following up genuine and experimental inflammatory dermatoses. The opportunities of 20 MHz ultrasonic imaging for quantitative assessment of early and late postradiation skin reactions are investigated. METHODS Between April and November 1996, 96 high resolution ultrasound examinations of the skin in 29 patients treated for breast cancer at the University of Ulm were analyzed. Total doses between 46 and 60 Gy were applied. The time interval between the completion of radiotherapy and ultrasonic examination was < or =3 months in 18 patients and 6-135 months in 11 patients. For examinations we used a digital high resolution ultrasonic system with a ceramic 20 MHz transducer. Irradiated and non-irradiated skin were compared. RESULTS A change of thickness and texture of the dermis depending on the time interval between the completion of radiotherapy and ultrasonic examination and on the administered radiation dose was found. There were significant differences between irradiated and non-irradiated skin regarding the dermal thickness in early (P < 0.001) as well as in late (P = 0.0018) reactions. Echogenicity of the upper and lower corium of irradiated skin decreased in early and late reaction. In upper corium the greatest reduction of signal intensity occurred in early reactions (P = 0.0001). Early reactions of the lower corium differed significantly from late changes (P = 0.001). Discrepancies between visible skin reactions described by examining physicians and ultrasonically proven changes were obvious mainly in late reactions. CONCLUSIONS There are specific textures of early and late postradiation skin reactions in comparison to non-irradiated skin. High resolution digital 20 MHz ultrasound is non-invasive and quantitative, and in contrast to physical examination, an easy reproducible method for assessing and documenting early and late skin reaction during and after radiation therapy treatment.