Quantitative ultrasonic characterization yields information that is correlated to the tissue microstructure and increases the diagnostic potential of ultrasound. The measurement of acoustic properties of melanomas in vivo has not yet been reported after proton-beam irradiation. This prospective study was conducted on a cohort of 50 patients diagnosed with primary malignant melanoma to assess in vivo the ability of quantitative echography to detect changes in choroidal malignant melanomas after proton-beam irradiation and to follow the ultrasonographic changes during 24 months posttreatment. Echographic evaluations of these patients were performed at diagnosis and repeated every 6 months after treatment over a 2-y period. The acoustic parameters included in this work are derived from the calibrated tissue backscatter spectra (spectral slope, spectral intercept and apparent integrated backscatter) of a selected tumor volume after correction of the apparatus transfer function and beam diffraction. Clinical parameters resulting from conventional echography were also quantified and included mainly tumor height, tumor vascularity and internal reflectivity. Spectral intercept and apparent integrated backscatter were found to be the most useful to evaluate changes in melanomas after treatment. Significant (p < 0.05) differences of these parameter values were observed between pre- and postproton therapy. In particular, significant changes (compared with baseline) were observed for these parameters, even when the tumor size after treatment was not significantly different from baseline. The results suggest that quantitative spectrum analysis of frequency-dependent backscatter can provide information about the structural modifications of choroidal malignant melanomas as a result of proton-beam irradiation.