Both the immediate and long-term effects of chest physical therapy administered by a parent were evaluated in eight children with cystic fibrosis. Spirometric and plethysmographic evaluations were performed pre-CPT and at 5 and 30 minutes post-CPT. The pre-CPT measurements after a three-week period with no CPT were compared with the values while receiving CPT on a regular twice daily basis. There was a significant decrease after three weeks without CPT for FVC (P less than 0.025), FEV1 (P less than 0.005), FEF25-75 (P less than 0.005), and Vmax60TLC (P less than 0.025). When the patients had been receiving CPT on a regular basis, the only immediate effect was an increase in PEFR after 30 minutes post-CPT (P less than 0.05). After three weeks without CPT, there were increases at 30 minutes post-CPI for FVC (P less than 0.005) and Vmax60TLC (P less than 0.05). These findings indicate that although there may be little immediate functional improvement when CPT is received on a regular basis, a three-week period without CPT leads to a worsening of the functional status, which is reversed with renewal of regular CPT.