Two hundre seventeen small malignant melanomas, each with a volume less than 1,400/cu mm, were studied to determine what factors were useful in predicting metastasis. Using a single-factor approach with 16 risk factors, we found seven that correlated well with outcome. These were cell type, pigmentation, size (largest dimension), scleral extension, mitotic activity, location of anterior margin of the tumor, and optic nerve extension. Using a linear discriminant function, the four best factors in combination were cell type, largest dimension, scleral extension, and mitotic activity. Four variables (largest diameter, location of anterior margin, mitotic activity, and optic nerve invasion) that might be correlated with clinical observations were found to be less accurate in separating fatal and nonfatal cases than cell type alone.