Discrimination between borderline and malignant mucinous ovarian tumours is a well-known diagnostic problem. In order to obtain objective reproducible and consistent features for differential diagnosis, 32 quantitative microscopical features were assessed in 10 benign, 10 borderline and 22 malignant mucinous ovarian tumours. There were many significant differences between the three groups, but using multivariate analysis there was 93% agreement between the histopathological assessment of these sections and the qualitative analyses. The following features were useful in the quantitative classification: the mean area, the mean perimeter and the mean of short axis of the nucleus; the volume percentage of the epithelium; the mitotic activity. In three cases, there was a difference between the original histopathological and computer classification. It was debatable whether the original diagnosis was correct, and therefore, all the cases were independently reassessed blind by three pathologists. Their diagnoses lend strong support to the computer classification in two of the three cases. The computer classification seems therefore to be even better than 93%. The present quantitative techniques are inexpensive, relatively easy to use, and, we believe, have a useful place in diagnostic histopathology.