A study of variations in the number of chromosomes in the fetal human brain by multicolor fluorescent in situ hybridization (mFISH) was carried out. A set of chro-mosome-specific DNA probes for chromosomes 1, 13, 18, 21, X, and Y was applied. Ten organotypic cell cultures of human fetal brains (material from medical abortions at 9-11 weeks gestation) were analyzed. One thousand hybrid­ized cells were scored for each sample. The level of aneu­ploidy (per individual chromosome pair) was in range of 0.2 to 15.6% in fetal brain cells. The mean frequencies of aneuploidies were 5% for chromosome 1; 3.7% for chro­mosomes 13 and 21; 1.8% for chromosome 18; 5.8% for chromosome X and 5.7% for chromosome Y. More com­mon aneuploidies were associated with aberrations involv­ing sex chromosomes and chromosome 1. The mean fre­quency of aneuplodies involving chromosomes 1, 13, 18, 21, X and Y was 22%. These data indicate that the fetal neuronal cells are characterized by a high frequency of aneuploidy. Large-scale genomic variations due to chro­mosomal complement instability in developing neuronal cells can have a substantial effect on brain development. Aneuploidy and “cryptic” chromosomal mosaicism in the human brain may have relevance to brain cancer and many neurological or psychiatric diseases.

Key words: Fetal human brain; Multicolor fluorescent in situ hybridization (mFISH); Chromosomes; Aneu­ploidy.