Twenty to 40% of Stage I(T1N0M0) cancers of the breast recur in ten years. This is an attempt to identify those patients in whom the disease is likely to recur. On the basis of a study of the histologic changes in the tumor and treatment failures poor prognosis was associated with several histological characteristics: poor cytologic differentiation; lymphatic permeation; blood vessel invasion and invasion of the tumor into the surrounding soft tissue. This classification was then applied to 363 cancers of the breast seen over a five year period and followed three to eight years. There were 203 Stage I (T1N0M0) tumors in the group. Ninety-four of the 203 Stage I tumors had one to four of the above histologic characteristics; 109 had none. Among the 109 patients characterized as good risks there were two treatment failures (2%). In the group of 94 with any high risk histologic features there were 47 treatment failures (50%) which were statistically significant (p = 0.001). The histologic changes had a cumulative effect on the degree of malignancy of the tumor. Pathologic changes in the tumor identified those patients whose Stage I (T1N0M0) tumors were likely to recur.