From the results of recent medical examinations, the authors investigated the relation between the health of benzen workers and the environmental concentration. The results are as follows 1) The specific gravities of blood (GB) and plasma (GP) were significantly influenced by the environmental benzen concentration. GB was significantly decreased in above 60 p. p. m. but not decreased in below 60 p. p. m.. GP was decreased in large concentration but increased in small concentration. 2) With the increase of the environmental concentration, the white cell counts were significantly decreased. 3) The red cell counts were not significantly decreased in below 20 p. p. m., but clearly decreased in above 60 p. p. m.. 4) The hematocrit values were significantly decreased in above 60 p. p. m.. 5) The coproporphyrine excretion in the urine was not influenced by the environmental benzene concentration. 6) The benzene obstacles was not influenced by the period of work, but it appeared early in large concentration and lately appeared in small concentration.