Data are presented on 115 children, including three sets of twins, born in Canada in 1961 and 1962 with congenital malformations associated with the use of thalidomide by their mothers in early pregnancy. The epidemiological method is described. Of the 115 children, 74 were alive at the time of reporting, 41 of these being severely handicapped; 8 had been stillborn; 33 had died. Limb involvement was usually bilateral, affecting the upper limbs alone in 42 cases, and the upper and lower limbs in 41. Of 112 mothers, only 60 had had the drug prescribed by the physician providing maternity care; 87 were estimated to have first taken the drug before their last menstrual period or within 56 days thereafter. A plea is made for the development of better methods of collecting information on the occurrence of congenital malformations, with the aid of practising physicians.