We report on a 22 year old man with hyperpigmentation distributed along the lines of Blaschko in whom cytogenetic analysis showed mosaicism for an unusual supernumerary marker chromosome. The patient was of normal intelligence and was not dysmorphic. The marker was present in 30% of his lymphocytes and in 6% of his skin fibroblasts from a dark area, while fibroblasts from a light area showed a normal karyotype, 46,XY. We have identified the origin of the marker using fluorescence in situ hybridisation (FISH) with whole chromosome painting probes and YAC specific clones. The marker was found to consist of duplicated chromosome material from the distal part of chromosome 3q and was interpreted as inv dup(3)(qter-->q27.1::q27.1-->qter). Hence, this marker did not include any known centromeric region and no alpha satellite DNA could be detected at the site of the primary constriction. The patient was therefore tetrasomic for 3q27-q29 in the cells containing the marker chromosome. We postulate that, in our case, pigmentary anomalies may result directly from the gain of specific pigmentation genes localised on chromosome 3q.