Objective: To describe levels, monthly variations and trends in weight and arm circumference of non-pregnant lactating women living in the Sahel, characterized by one short yearly rainy season (July–October). Methods: A mixed unbalanced cross-sectional longitudinal observational study conducted at 3, 5, 7 and 10 months postpartum among 3869 women living in the Sine area in central Senegal who had brought their infants into dispensaries for immunization from January 1990 to February 1997, and 1–5 consecutive children per woman (26 106 visits). Results: Mean weight was 55.7 kg (s.d.: 7.1), but it varied by 2.5–3.9 kg each year, from high means during the dry season (March–May) to low means at the end of the rainy season (September–November). The prevalence of underweight, overweight and obesity (body mass index (BMI)<18.5, 25–29.9 and >30 kg/m2, respectively) was 7.6% (95% confidence interval: 7.3, 7.9), 6.4% (6.1, 6.7) and 0.4% (0.3, 0.4), but varied strongly by season (P<0.0001 for all). Unlike weight, mean arm increased during the early rains, a peak season of agricultural work (+0.10 cm/month (s.d.: 0.6) from June to August vs -0.35 kg/month (s.d.: 1.1) for weight). BMI and arm circumference were positively associated with age (mean: 20.8 vs 22.2 kg/m2 and 25.3 vs 27.4 cm, at 20–24 and 40–49 years, respectively, P<0.0001 for both). Conclusions: Season was a major determinant of the anthropometric status of rural African women. Negative energy balance reduced body weight from the onset of agricultural labour, while arm circumference increased during early rains, probably due to high physical activity.