We report here a database listing Mendelian phenotypes described in the Netherlands and/or in populations originating from this country, and describe the results of a quantitative analysis of the database. The database is specifically directed at the presence, frequency and origin of the phenotypes. These are arranged according to their mode of inheritance: autosomal dominant (AD), autosomal recessive (AR) and X-linked. Only those phenotypes which have been reported in accessible sources were included. We entered 1,482 references up to January 1, 1991. At least 672 different loci were described in the Netherlands at this date: 321 (47.8%) AD, 283 (42.1%) AR and 68 (10.1%) X-linked. Almost 2.5% of all loci in our database have no comparable entry in McKusick [mendelian Inheritance in Man, ed 9. Baltimore, The Johns Hopkins University Press, 1990] (MIM). There is a significant difference (p < 0.01) according to the division into AD, AR, and X-linked phenotypes between our database and MIM, in which 61.7% of the phenotypes are AD, 31.5% AR and 6.8% X-linked. Dutch prevalence data for 38 monogenic disorders and 24 polymorphic systems are listed.