Monosomy 1p36 is one of the most commonly observed mental retardation (MR) syndromes that results in a clinically recognizable phenotype including delayed psychomotor development and/or MR, hypotonia, epilepsy, hearing loss, growth delay, microcephaly, deep-set eyes, flat nasal bridge and pointed chin. Besides, a Prader-Willi syndrome (PWS)-like phenotype has been described in patients with 1p36 monosomy. Forty-one patients presenting hypotonia, developmental delay, obesity and/or hyperphagia and behavioral problems who tested negative for PWS were investigated by FISH and/or microsatellite markers. Twenty-six were analyzed with a 1p-specific subtelomeric probe, and one terminal deletion was identified. Thirty patients (15 of which also studied by FISH) were investigated by microsatellite markers, and no interstitial 1p36 deletion was found. Our patient presenting the 1p36 deletion did not have the striking features of this monosomy, but her clinical and behavioral features were quite similar to those observed in patients with PWS, except for the presence of normal sucking at birth. The extent of the deletion could be limited to the most terminal 2.5 Mb of 1p36, within the chromosomal region 1p36.33-1p36.32, that is smaller than usually seen in monosomy 1p36 patients. Therefore, chromosome 1p36.33 deletion should be investigated in patients with hypotonia, developmental delay, obesity and/or hyperphagia and behavioral problems who test negative for PWS.