We report here on two mosaic patients with both an idic(Yp) and a microchromosome. FISH with the DYZ3 alphoid repeat demonstrated that the isodicentrics effectively exhibited two alphoid clusters whereas the small markers had a Y-centromere. These data, along with 4 previous observations, indicate that such microchromosomes effectively result from functional dicentricity of isodicentric Y-chromosomes and represent the excision of one centromere plus various amounts of adjacent chromatin. Other than a real infrequency of such a concurrence or a very low proportion of the cell line(s) containing the microchromosome, the paucity of observations points to a high rate of underdiagnosis as revealed by two idic(Y) instances in which the microchromosome was detected only in samples assessed by FISH.Instituto Mexicano del Seguro Social, División Genética, Guadalajara, Jalisco, Mexico