The invention provides methods and kits for diagnosing a growth in a subject by providing a sample of a growth taken from a subject, determining the level of midkine or pleiotrophin in the sample by an immunoassay, and comparing the level of midkine or pleiotrophin determined from the sample with a control. An increased level of midkine or pleiotrophin in the sample as compared to the control is diagnostic of a malignant growth, whereas an equivalent or decreased level of midkine or pleiotrophin in the sample as compared to the control is diagnostic of a benign growth.