Factors significantly associated with the risk of developing AIDS included current CD4 (aRR 0.50 per doubling of the count, 95% CI 0.40–0.62, p<0.001), diagnosis of anaemia at baseline (aRR 2.21, 95% CI 1.46–3.35, p<0.001), and a previous AIDS condition (different from that observed in follow-up, aRR 2.07, 95% CI 1.33–3.20, p = 0.001). This evidence concerns the gene CD4 and AIDS.