Nutritional rickets in Nigerian children has been effectively treated with Ca supplementation. High values of Ca absorption efficiency have been observed in untreated children, but whether Ca absorption efficiency changes during treatment with Ca is unknown. Our objective in conducting this study was to identify the effect of Ca therapy on Ca absorptive efficiency in children with primary Ca-deficient nutritional rickets. Twelve children with radiographically active rickets, 2 to 14 years of age (median 39 months) participated in the study. We assessed dietary Ca intake via dietary recalls, and measured biochemical markers of Ca and vitamin D homeostasis. Fractional Ca absorption was measured using a dual tracer stable isotope method, before and after 2 weeks of treatment with 15·0 mmol elemental Ca daily. Ten children had adequate urine collection for inclusion in the analysis. Usual dietary Ca intake was 4·2 (sd 1·0) mmol/d. The median Ca absorption prior to treatment was 72 % (range 52–97 %) and decreased significantly to 57 % (31–84 %) (P = 0·004) after 2 weeks of supplementation. We conclude that Nigerian children with rickets adapt to Ca supplementation with a small decrease in Ca absorptive capacity, but retain very high absorptive levels during supplementation. Overall Ca absorption efficiency was comparable with that identified in other populations with low Ca intakes. These data demonstrate that although absorptive capacity is regulated by supplementation, recovery from rickets likely occurs through efficient use of both dietary and supplemental Ca.