Haematopoiesis could be affected by mustard gas. We randomly selected 318 chemical victims exposed to mustard gas and compared their cell blood counts and peripheral blood smears (PBS) with those of 377 healthy men, and also various haematological indices of 57 of these victims compared with previous data 5 years ago. The average number of red blood cells and haemoglobin of victims compared with the controls was not significantly different, but they were increased compared with data from 5 years ago. White blood cell counts, neutrophils and lymphocytes did not show any clinically meaningful difference compared with the control group but 20 cases with atypical lymphocytes in their PBS have been found. Change in lymphocyte shape may be related to committed stem cell involvement. Further studies on bone marrow cells and cell markers are needed to document this hypothesis. The mild increase in erythroid cells and haemoglobin concentration may be due to chronic obstructive pulmonary disorder and other respiratory diseases in these patients.