The 5q-syndrome has been recognized as a distinct subtype of myelodysplastic syndrome (MDS) with characteristic clinical and pathologic features. Nevertheless, the definition of this syndrome is imprecise. To better understand how the 5q-syndrome is related to other "5q- only" myeloid disorders, we searched our conventional cytogenetics file for cases with 5q- as the sole karyotypic abnormality. 31 cases of "5q- only" myeloid disorders were found, and they were refractory anemia (n = 16), refractory anemia with excess blasts (RAEB) (n = 5), RAEB in transformation (n = 1), chronic myelomonocytic leukemia (n = 1) and acute myeloid leukemia (n = 8). They included 15 men and 16 women, with a median age of 67 years (range, 40-84 years). The marrow blast count was < or = 11% in 22 cases and > or = 25% in the remaining nine cases. The following morphologic features were common in the marrow: megakaryocytic hypolobation (30/31, 97%), erythroid hypoplasia (26/31, 84%), basophilia (19/31, 62%) and eosinophilia (16/31, 52%). Of those with < or = 11% blasts, 7/22 cases met the criteria of the 5q-syndrome, as defined by high mean corpuscle volume (MCV), normal/high platelet counts, and megakaryocytic hypolobation. Except for the two defining parameters for the 5q-syndrome (MCV and platelet count), there was no significant difference in hematologic parameters between the 5q-syndrome and other cases with < or = 11% blasts. Furthermore, no significant difference in the chromosomal breakpoints or survival was found between these two groups. We conclude that "5q- only" MDS show consistent morphologic features, suggesting a common pathogenesis related to their similar cytogenetic abnormality. In "5q- only" MDS with < or = 11% marrow blasts, strict application of the conventional criteria of the 5q-syndrome may not be necessary in predicting the overall prognosis.