twin sisters in massachusetts. they have been participating in the clinical trial of a cystic fibrosis drug. one sister taking the drug. the other hasn t. abc s dr. richard besser tells us more. reporter: two sisters with a rare form of cystic fibrosis. one gets better. the other doesn t. both part of a study for a drug that has the potential to dramatically change their lives. but as in most studies, only half the patients get the real drug. we didn t know if they were both getting sugar pill or both the drug, or one and one. the pills looked the same. within weeks the girls weren t acting the same. i wasn t coughing as much as laura was at that time. everybody said i was getting a lot more energy and growing better. she had a huge weight gain. her chest was clear. and the thing that blew us all away was that her lung function test skyrocketed. reporter: because of cystic fibrosis, kate and laura have damaged lungs, persistent violent coughing, and they don t grow as wel