A review of the diagnoses in cases with blood smears showing striking poikliocytosis was made over a 2-yr. period. After excluding renal failure and primary hematological disease, carcinoma was present in 17 out of 27 cases, and in 9 cases the primary site of the carcinoma was in the alimentary tract. The presence of marked poikilocytosis was of diagnostic significance and indicated a grave prognosis. Fourteen of the patients with carcinoma died only a few wk. after the abnormal blood film was 1st observed, and unlike cases reported previously thrombocytopenia and overt signs of hemolysls were rarely present. Leukoerythroblastosis was seen in 7 cases of carcinoma, and its significance was less serious than was supposed.