The tricho-rhino-phalangeal syndromes type I (TRPS I) and type II (TRPS II) result from the deletion of overlapping sets of genes within the Langer-Giedion syndrome chromosomal region (LGCR) on chromosome 8. In contrast to TRPS I patients, most TRPS II patients have cytogenetically visible deletions and are often mentally retarded. Using Southern blot and fluorescence in situ hybridization analysis, we searched for submicroscopic deletions in 12 patients with TRPS I and an apparently normal karyotype. One patient of normal intelligence was found to have a deletion of approximately 5 Mb. This suggests that mental retardation in TRPS is caused by genes outside the 5-Mb region. Using three LGCR microsatellite markers, we determined the parental origin of this TRPS I deletion and of eight TRPS II deletions. In six patients, the deletion was of paternal origin and in three patients it was of maternal origin.