The coexistence of child malnutrition and maternal overweight in the same households typifies rapid nutrition transition in developing countries (DCs). It is reportedly less common in Africa than in Latin America or Asia, but the phenomenon is still little documented. The purpose of our study in poor urban neighbourhoods of the capital city of Benin (West Africa) was to assess the magnitude of the overlap of child protein-energy malnutrition (PEM) and maternal overweight within households, and to compare these households with other nutritional phenotypes in terms of socio-economic circumstances and diet quality. Our hypothesis was that both child PEM and maternal overweight could stem from poor socio-economic conditions, including lack of sanitation, and poor diet quality. Food diversity was used as an index of diet quality, or the qualitative dimension of food security. A random sample of 148 households that included a least the biological mother, one child between 6 and 59 months of age and a second one between 5 and 11 years was selected in two poor neighbourhoods of the capital city of Cotonou to assess the prevalence of "double burden" households and of other nutritional phenotypes of households: with PEM only in at least one child; with maternal overweight only; and without PEM or overweight. Body weights and heights of mothers and of the two targeted children were measured. As long as one child had low weight-for-height or height-for-age (z-score <-2.0), the household came under the "PEM" type. In mothers, overweight was defined as body mass index (BMI) > or = 25, obesity > or = 30 and chronic energy deficiency <18.5. We retained 126 households for interviews with mothers on socio-economic circumstances and food diversity. A socio-economic status (SES) score was constructed on the basis of household amenities and maternal education. Food insecurity was based on reported shortage of food in the last year in the household. Based on the frequency of consumption of 13 different food groups in the previous week, a food diversity score was computed. Overall 35.5% of children were malnourished, and school-age children had a worse nutritional status than under-5 children: 41% and 30% PEM (chronic or acute or both), respectively. The rate of maternal overweight was 39.1% including 15.5% of obesity. Child PEM coexisted with maternal overweight or obesity in 16.2% of the households; 27.7% of households had PEM only, 23% overweight only, 20.3% showed no malnutrition or overweight, and 12.8% had an underweight mother. Maternal BMI status was significantly associated with both children's weight-for-height z-score, particularly the elder one. The rate of child malnutrition, particularly wasting, was significantly higher among underweight mothers and lower in overweight mothers . Underweight mothers were merged with mothers with BMI < or = 25 for the remaining analyses. Households with overweight mothers tended to enjoy relatively better socio-economic conditions--higher SES, higher maternal education, less food insecurity, better household sanitation; they also tended to have a more diversified diet. This is in contrast with PEM households. Dual burden households shared several socio-economic features with the PEM households, except for a higher (not significant) SES score. Dual burden households also had the lowest food diversity score of all household types. Logistical regression models revealed that a relatively higher SES level was associated with a higher likelihood of maternal overweight in PEM households, whereas poor household sanitation increased the odds of PEM among maternal overweight households. Food diversity appeared significantly associated with a lower likelihood of dual burden in all types of households. The study highlights the importance of addressing the double burden of malnutrition and overweight even in poor areas of low income countries of West Africa. It suggests that prevention efforts should be aimed at improving diet quality and sanitation in poor urban households.