Low bone density is a complication of cystic fibrosis (CF). Accrual of bone mass is most impaired in the sickest children, as judged by nutritional status and pulmonary function. Retrospective analysis of correlation between lumbar spine bone mineral density (BMD), body mass index (BMI), and forced expiratory volume in 1 second (FEV1) z scores in children and adolescents with CF. Pediatric hospital specialist CF service. Sixty participants aged 5.9 to 18.8 years (24 female) with confirmed CF. Lumbar spine BMD, BMI, and FEV1 z scores measured at first BMD scan; 40 participants had sequential scans. Change in L1-L4 z scores over time was used as a measure of bone accrual, and BMI as a measure of nutritional status. Correlations between lumbar spine BMD, BMI, and FEV1 z scores. Mean BMI and BMD z scores were strongly correlated at the initial scan (P < 0.0001), suggesting that nutritional status is a major determinant of BMD. In the sequential scan at a mean age of 16.1 years, height centile was maintained, indicating normal linear growth. Changes in BMI and BMD z scores were positively correlated (P = 0.001), indicating that patients failing to gain weight appropriately with growth were also failing to acquire bone normally. Change in FEV1 z score was correlated with change in both BMD (P < 0.0001) and BMI z scores (P = 0.02). Although young people with CF may be maintaining normal growth in height, bone accrual is impaired in those with the poorest nutritional status and lung function.