Aims: To compare ultrasonographic (US) predicting factors for conversion of choroidal naevi into melanomas. Methods: 659 consecutive eyes with choroidal naevi were examined between 1984 and 2004. 165 clinically suspicious naevi were followed clinically and ultrasonographically (thickness, base diameters, internal reflectivity and location in the eye) for 5.08 (SE 0.24) years. Results: 17 naevi (2.6%of all naevi, 10.3%of suspicious naevi) converted to small choroidal melanomas. The thickness of benign and premalignant naevi differed significant only after 1.5 years of follow up. The mean initial thickness of benign and premalignant naevi was significantly different (p=0.001), as was mean initial internal reflectivity (p=0.002) and mean initial largest base diameter (LBD, p=0.05). Posterior pole and nasally located naevi were more likely to become malignant. A thickness of ≥7 mm and a LBD ≥7 mm were most predictive of conversion to melanoma, as was a combined K1 index of ≥14.5 (K1=LBD+4 x thickness +1 (for nasal location) +1 (for posterior pole location)). An artificial neural network did not have a better forecasting accuracy than the K1 index. Logistic regression found the only significant parameters to influence the risk of conversion to melanoma to be the K1 value and the initial tumour thickness. Conclusions: A follow up of at least 1.5 years is necessary to detect conversion of naevi to choroidal melanomas. The thickness and LBD of the lesion can be used for predicting the risk. Methods: 659 consecutive eyes with choroidal naevi were examined between 1984 and 2004. 165 clinically suspicious naevi were followed clinically and ultrasonographically (thickness, base diameters, internal reflectivity Results: 17 naevi (2.6% of all naevi, 10.3% of suspicious naevi) converted to small choroidal melanomas. The thickness of benign and premalignant naevi differed significant only after 1.5 years of follow up. The mean initial thickness of benign and premalignant naevi was significantly different (p = 0.001), was was initial initial reflectivity (p = 0.002) and mean initial largest base diameter (LBD, p = 0.05) A thickness of ≥7 mm and a LBD ≥7 mm were most predictive of conversion to melanoma, as was was combined with K1 index of ≥14.5 (K1 = LBD + 4 x thickness +1 (for nasal location) +1 (for posterior pole location)). An artificial neural network did not have a better forecasting accuracy than the K1 index. Logistic regression found the only significant parameters to influence the risk of conversion to melanoma to be the K1 value and the initial tumor thickness. A Conclusions: A follow up of at least 1.5 years is necessary to detect conversion of naevi to choroidal melanomas. The thickness and LBD of the lesion can be used for predicting the risk.