BACKGROUND--Exposure to high levels of benzene commonly results in the suppression of hemopoiesis, although cases of leukocytosis and leukocytosis with thrombocytosis have been reported. No hematologic abnormalities have generally been found with exposure to low levels of benzene. METHODS: A pipe fitter exposed to low levels of benzene (time-weighted average 0.9 ppm) developed leukocytosis. His blood counts and growth of erythroid burst forming units (BFU-E) was followed with and without the addition of erythropoietin. RESULTS: Erythropoietin-independent BFU-E colonies were increased to 40 per 4 x 10(4) cells (normal < 3 per 4 x 10(4) cells). Both the leukocyte count and the number of erythropoietin-independent BFU-E colonies decreased when exposure to benzene was terminated. On reexposure the white blood count again increased. After the work was terminated, the white blood count returned to normal, as did the number of erythropoietin-independent BFU-E colonies, over a period of 12 months. CONCLUSIONS: Our findings suggest that even low levels of benzene can result in perturbations of the hemopoietic system. Further studies are warranted to determine whether these findings are idiosyncratic, coincidental, or a more general phenomenon.