Uniparental disomy (UPD) describes the inheritance of a pair of chromosomes from only one parent. Meiotic nondisjunction followed by trisomy rescue is considered to be the major mechanism of formation. A literature search for cases with whole chromosome UPD other than UPD 15 was performed. Information on parental age was available in 111 cases with maternal UPD and in 34 cases with paternal UPD. In 52 out of 74 cases with maternal heterodisomy, information on the time of nondisjunction was also available. Around two-thirds of these cases were due to a maternal meiosis I error. Compared with the mean maternal age of 30.0 years in Bavarian mothers, in the year 2000 an advanced mean maternal age of 34.8 years was found in cases with maternal heterodisomy (n=74; P<0.0001). Almost no difference in the mean maternal age was observed between meiosis I errors (35.56 years; n=30) and meiosis II errors (35.78 years; n=14). The mean maternal age was 31.46 years in cases with maternal isodisomy and a normal karyotype (n=24), and the mean paternal age was 31.48 years in cases with paternal isodisomy (n=28). The various mean parental ages in heterodisomic and isodisomic cases are considered to reflect strongly the different mechanisms of formation: trisomy rescue or gamete complementation, which implies a meiotic nondisjunction in maternal heterodisomic UPD, and postzygotic somatic reduplication in cases with paternal and maternal isodisomic UPD.