Cytogenetic studies after short-term culture were performed on 32 adenocarcinomas of the prostate from patients without prior treatment. The tumor specimens, ranging from stage B1 to D1, were obtained by radical prostatectomy or diagnostic biopsies. Fourteen tumors showed a normal diploid chromosome complement in all metaphases examined. Clonal chromosomal alterations were detected in 16 tumor samples and the remaining two cases contained double minute (dmin) chromosomes in some cells. The most frequent numerical changes included loss the Y chromosome and trisomy 7, both found in four cases. The only recurrent structural aberration was del(10)(q24), seen in three cases both as a sole anomaly and within multiple rearrangements. Six patients showed cytogenetically unrelated clones. The occurrence of the chromosomal changes found in this study shows no relationship to certain histopathologic characteristics of the tumors. The recurrent finding of del(10)(q24) as sole anomaly and the evidence for clonal evolution in one patient demonstrates that this change is an early karyotypic event which may be important for the pathogenesis in at least a subset of prostatic cancers.