Unique rounded granules consisting of clustered parallel tubules, 29-31 nm in diameter, with occasional, flocculent or amorphous electron-dense material (parallel tubular granules, PTGs) were found in immature neutrophils in the bone marrow in a case of chronic neutrophilic leukemia (CNL). PTGs were positive for electron microscopic myeloperoxidase. Since no structures similar to PTGs have been documented, we investigated the bone marrow of 65 adult patients with hematologic diseases, and 7 adult patients with miscellaneous non-hematologic diseases, by electron microscopy. The results showed that no PTGs were found in any cases other than the original CNL case. These findings suggest that PTGs are rare granules representing certain primary granules. In addition, the presence of PTGs might be helpful in diagnosing CNL.