Background and objective <p>Children with sickle cell disease (SCD) frequently experience chronic inflammation, increased metabolic demands, and recurrent acute complications. Asthma is a recognised comorbidity associated with increased morbidity in SCD. However, the contribution of nutritional status, particularly body mass index (BMI), to clinical severity among children with SCD and confirmed asthma remains poorly documented, especially in tropical settings.</p> <p>The objective of this study was to describe the nutritional status of children with SCD and spirometry-confirmed asthma and to assess the association between BMI-for-age z-scores and clinical severity.</p> Methods <p>We conducted a multicentre observational study including children aged 5–17 years with SCD and spirometry-confirmed asthma. Nutritional status was assessed using WHO 2007 BMI-for-age z-scores calculated from measured weight and height. Undernutrition was defined as a BMI-for-age z-score &lt; − 2. Clinical severity was defined as the occurrence of at least two hospitalisations for vaso-occlusive crises and/or acute chest syndrome in the preceding 12 months. Associations between nutritional indicators and clinical severity were evaluated using bivariate and multivariable logistic regression models adjusted for relevant clinical covariates.</p> Results <p>A total of 138 children were included (median age 8.0 years). Overall, 17.4% presented undernutrition, while 12.3% were overweight or obese. In bivariate analyses, undernutrition was more frequent among children with severe disease than among those with non-severe disease (24.5% vs. 13.5%). When BMI-for-age z-score was analysed as a continuous variable, lower values were associated with increased odds of severe disease (OR per 1-SD decrease = 1.34; 95% CI 1.01–1.78). After multivariable adjustment, the association between BMI-for-age z-score and severity was attenuated and did not reach statistical significance (adjusted OR 1.29; 95% CI 0.96–1.74; <i>p</i> = 0.09).</p> Conclusion <p>These findings highlight the contribution of clinical and inflammatory factors to disease severity in children with sickle cell disease and confirmed asthma, including prior ACS, which showed a borderline association. Clinical severity appeared to be more closely related to respiratory burden than to nutritional status alone. Lower BMI-for-age z-scores reflected global systemic vulnerability rather than an independent risk factor. Routine nutritional assessment may therefore support risk stratification and help identify children who could benefit from targeted multidisciplinary care.</p>