In acute myeloid leukemia (AML) close associations are known between cytomorphology and cytogenetics such as in AML M3/M3v showing a t(15;17) and in AML M4eo associated with inv(16)/t(16;16). In AML M5 a heterogenous cytogenetic pattern is observed. We describe the gain of an isochromosome of the short arm of chromosome 5 together with the gain of chromosome 8 as the sole abnormalities in two cases of acute monoblastic leukemia. In a third case of acute monoblastic leukemia we also observed the gain of an isochromosome 5p together with trisomy 8. This patient showed in addition an unbalanced translocation between the long arm of chromosome 1 and the short arm of chromosome 14 leading to a trisomy 1q. So far only two cases of AML with i(5)(p10) have been published. In no other hematological malignancy has an isochromosome 5p been reported up to now. As an isochromosome 5p can be misinterpreted as a deletion 5q, which occurs frequently in AML, fluorescence in situ hybridization with loci specific probes is a helpful method to detect this rare abnormality.