The treatment regime to be adopted for patients who have been diagnosed as having cancer of the prostate is crucially dependent upon whether or not the cancer has spread to the surrounding lymph nodes. Indeed, a laparotomy (a surgical incision into the abdominal cavity) may be performed to ascertain the extent of this nodal involvement. There are a number of variables that are indicative of nodal involvement which can be measured without the need for surgery, and the aim of a study reported by Brown (1980) was to determine whether a combination of five variables could be used to forecast whether or not the cancer has spread to the lymph nodes. The five variables were: age of patient at diagnosis (in years), level of serum acid phosphates (in King-Armstrong units), the result of an X-ray examination (0=negative, 1=positive), the size of the tumour as determined by a rectal examination (0=small, 1=large) and a summary of the pathological grade of the tumour determined from a biopsy (0=less serious, 1=more serious). The values of each of these five variables were obtained from 53 patients presenting with prostate cancer who had also undergone a laparotomy.