A newborn girl with generalized muscular hypotonia and minor anomalies was referred for chromosome analysis. Cytogenetic investigation showed a satellite and an Ag-positive NOR on the distal short arm of one chromosome 1, thus indicating an unbalanced translocation involving the short arm of an acrocentric chromosome. The phenotypically normal mother had the same satellited chromosome 1 with Ag-positive NOR. One chromosome 15 was the only acrocentric chromosome in her karyotype lacking recognizable satellites and an Ag-positive NOR. Thus a balanced reciprocal translocation between the short arms of chromosomes 1 and 15 in the mother was suggested. The cytogenetic diagnosis was confirmed by nonradioactive in situ hybridization with the most distal DNA probe on chromosome 1, the probe p1-79, localized at chromosome band 1p36.3. The probe was biotinylated by nick-translation, and detection was done by FITC labelled avidin binding. Hybridization signals were observed on both the mother's normal chromosome 1 and the derivative chromosome 15 but not on her derivative chromosome 1. Consequently, the index patient has an unbalanced karyotype with monosomy (1p36.3). Comparing their clinical reports shows that patients with similar terminal deletions of 1p share several manifestations.