OBJECTIVE: To investigate the distribution of overweight and obesity and its relationship with socio-economic and behavioural factors in a developing-country population undergoing rapid nutritional transition. DESIGN: Cross-sectional house-to-house survey in urban Gambia. SUBJECTS: Four groups of 50 subjects were sampled as follows: young men (YM, 14-25 years), young women (YW, 14-25 years), older men (OM, 35-50 years) and older women (OW, 35-50 years). MEASUREMENTS: Several socio-economic and behavioural factors were investigated. Composite indices for socio-economic status, education, healthy lifestyle and western influences were created. Body weight, height, waist and hip circumferences were measured and body mass index (BMI) was calculated. Body composition was assessed by leg-to-leg bioimpedance. Overweight was defined as BMI=25.0-29.9 kg/m(2) and obesity as BMI>or=30.0 kg/m(2). RESULTS: There were highly significant gender and age differences in overweight (YM=0%, YW=10%, OM=6% and OW=34%) and obesity (YM=0%, YW=4%, OM=6% and OW=50%). Only 16% of OW were neither overweight nor obese compared to 88% of OM. OW had a higher fat mass percent (38.4%) than other groups, while fat-free mass (kg) was significantly higher in males than females with YW having the lowest value. Young generations were more educated and more influenced by western ideals than OM and OW. Weight gain was not always associated with weight concern and many overweight/obese subjects did not perceive themselves as overweight. CONCLUSION: Social and behavioural changes are already creating a perceptible 'generational gap' among this population undergoing rapid transition. The improved education and current lean status of the younger adults offers opportunities for preventative interventions. These need to be specially targeted at women.