We looked for correlations between cytogenetic rearrangements leading to 17p deletion and presence of dysgranulopoïesis and p53 mutations in MDS and AML. Forty-nine (4.3%) of the MDS and AML studied cytogenetically at our institution over a period of 11 years had detectable 17p deletion, through monosomy 17 (14 cases) or rearrangements of chromosome 17 (generally unbalanced translocations between 17p and another chromosome) (35 cases). Most of the patients had additional complex cytogenetic findings, and 10 cases were therapy related. In 70% of the patients with 17p deletion, a particular type of dysgranulopoïesis, combining pseudo-Pelger-Huët anomaly and small vacuolated neutrophils was seen in > 5% marrow neutrophils, whereas 69% of the patients had a p53 mutation, generally in a missense mutation involving exons 5 to 8 of the p53 gene. FISH analysis, performed in eight cases, confirmed loss of one P53 allele in all of them. No DNA fragmentation suggesting increased apoptosis was found in marrow samples. Response to chemotherapy was almost uniformly poor and median survival was only 3 months. Analysis of dysgranulopoïesis and p53 mutations were also made in 'control' groups of MDS and AML without 17p deletion. 'Typical' dysgranulopoïesis, combining pseudo-Pelger-Huët anomaly and small vacuolated neutrophils in > 5% marrow neutrophils, was not seen in any of the 47 MDS and AML without 17p deletion analyzed and without p53 mutation (P = 10(-4) with patients having 17p deletion), and was seen in one of five patients without 17p deletion but with a p53 mutation. Only 3.1% of 256 MDS and AML without 17p deletion had a p53 mutation (P = 10(-4) with patients having 17p deletion). These findings suggest that 17p deletion, in MDS and AML, is strongly correlated to the presence of a particular type of dysgranulopoïesis and to a high incidence of p53 mutations, and that MDS and AML with 17p deletion could constitute a new morphological-cytogenetic-molecular entity in myeloid disorders.