A classical twin study is one of the best methods to address the open question of the role of genes and environment in the cause of cleft lip with or without cleft palate [CL(P)]. In addition, when twin concordance rates are combined with information about the risk for CL(P) to more remote relatives, they can help to establish the most likely mode of inheritance for CL(P). The present study was based on three nation wide ascertainment sources of CL(P) in twins in Denmark during the period 1970-1990. The Danish surgical files were found suitable for ascertaining twins with non-syndromic CL(P) and a total of 39 pairs was identified through these files. In more than 70% of the cases, the zygosity assignment was based on unlike-sex or an extensive panel of blood, serum, and enzyme types. More mono- and dizygotic twin (CL(P) cases than expected were found, although the difference was not significant. The proband concordance rate for CL(P) was 60% in monozygotic twins and 10% in dizygotic twins; six cases were of unknown zygosity. This finding indicates that genetic factors play a major role in the cause of CL(P) but environmental and/or stochastic factors are probably acting too. When the familial recurrence patterns analyzed by Mitchell and Risch [1992: Am J Hum Genet 51:323-332] are interpreted in light of these new estimates of monozygotic concordance, they provide further evidence for the finding that no single locus can account for more than a six-fold increase in risk to first-degree relatives.