A six-month pilot study of variable weight training (VWT) was undertaken to assess its impact on body weight, pulmonary function, muscle size and strength, and social function in 12 adolescent and adult patients with moderately severe cystic fibrosis. Exercise for patients with cystic fibrosis (CF) has often been recommended as an adjunct to physical therapy, although aerobic exercise has not resulted in weight gain in CF. Compared to a three-month control period, six months of VWT resulted in significant increase in weight (2.88 kg, p less than .02), muscle size (1.6 to 1.8 cm upper arm, p less than .01), strength (increase from 16 to 32 muscle groups at normal strength, p less than .005), and decrease in residual volume (1.77 L, p less than .03) and RV/TLC (12.4 percent, p less than .02). There was no significant improvement in other measures of pulmonary or social function. VWT appears to be a form of exercise in which even moderately ill CF patients can engage safely, leading to desired weight gain and increased strength. These results warrant further study of the effects of VWT on pulmonary function and CF morbidity.