The current strategy for the interruption of transmission of lymphatic filariasis in areas where the disease is co-endemic with onchocerciasis is repeated annual mass treatment of endemic communities with ivermectin and albendazole. These drugs are not recommended for use in pregnancy. Pregnant women are excluded on the basis of their last menses. This exclusion criterion based on recall carries some inherent errors, leading sometimes to inadvertent exposure of foetuses to these drugs. This study set out to document the extent of inadvertent exposure of pregnant women to albendazole and ivermectin and assess the relative risk of congenital malformations because of inadvertent treatment with these drugs in early pregnancy. The study was conducted in the Ahanta West District of Ghana. Local pregnancy revelation norms were studied, followed by a household survey of women aged 15-45 years to assess drug administration coverage. All infants born within 42 weeks of the mass drug treatment were examined to document any congenital malformations. Mothers who had lost any such infants responded to a verbal autopsy to ascertain the probable cause of death. Health facilities and local Traditional Birth Attendants were also visited to review maternity records. Of 2985 women of childbearing age (15-49 years) who were interviewed, 343 were pregnant during the mass drug administration. The sensitivity of the last menstrual period in detecting pregnancy and thus being excluded from treatment was 0.854 (293 of 343). Some pregnant women 50 of 343 (14.6%) had thus been inadvertently treated. This represents 1.7% of women in fertile age group (15-49 years). Of the six children found with some congenital malformations in these communities, one had been exposed to the drugs in-utero. The relative risk for congenital malformation after exposure was 1.05 (P=1.0). Two of nine reported spontaneous abortions had been exposed to the drugs (P=0.62). We conclude that the local mode of excluding pregnancy in the current programme, while not perfect, is sufficiently effective and reliable for such a public health intervention; and importantly, that there is no evidence of a higher risk of congenital malformation or abortions in those who are inadvertently exposed.