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High frequency sonography has been shown to be a useful tool in the preoperative staging of malignant melanoma. In the present study sonometric and histometric data concerning tumour thickness were compared, using appropriate statistical methods, in order to assess the accuracy of ultrasonography. From December 1997 all pigmented lesions suspected of being melanoma were preoperatively assessed by a 20 MHz ultrasound B scan. The results of these ultrasound examinations were compared with histometric data. Pearson's correlation coefficient and absolute and relative differences were used for statistical analysis. Of the 261 examined lesions, 193 were malignant melanoma. A high correlation between sonometry and histometry was computed (r = 0.95), with an absolute difference of 0.32 ± 0.03 mm (mean ± SEM) and a mean relative difference of 27.2% (95% confidence interval 23–31.4%). The highest correlation was found in melanoma ≥ 1.51 mm thick and the lowest correlation in melanoma ≤ 0.75 mm. In conclusion, the high accuracy of this technique in the preoperative staging of malignant melanoma would offer a basis for defining the surgical margins of ≥ 0.76 mm thick lesions. The limited accuracy of sonometry in the preoperative staging of thin melanoma ≤ 0.75 mm has emerged by applying adequate statistical methods.