Nebulized antibiotic therapy in cystic fibrosis is an established procedure. The present study was designed to quantitate deposition, and assess its relation to the disease state. Twenty seven children and young adults with cystic fibrosis (mean 11.6 years, range 4-23 years, 12 females) were studied to establish the quantity and pattern of deposition of nebulized tobramycin in the respiratory tract. A single (120 mg) dose of nebulized 99 m technetium-labelled tobramycin was administered, and imaged with a gamma-camera. The mean penetration index (which compares the distribution of 81 m-Krypton gas with Tc-radioaerosol) was also used to measure peripheral deposition efficiency. The aerosol mass median diameter (MMAD) for the compressor-nebulizer system used was 5.3 u, measured with the Malvern Mastersizer. Serial sputum samples were fluroimmunoassayed for tobramycin in nine patients. A mean of 8.0 (SEM 1.0) mg tobramycin reached the lungs. There was no relationship between the total pulmonary deposition and indices of pulmonary damage in cystic fibrosis. Sixteen percent of the lung tobramycin reached the periphery. The greater the lung damage as indicated by FEV1 and Chrispin-Norman scores, the smaller the proportion of pulmonary tobramycin that reached the periphery. The mean penetration index increased with increase in the FRC, but bore no relation to other respiratory function tests or to chest X-ray scores. Sputum tobramycin concentrations reached levels bactericidal for Pseudomonas aeruginosa. Airway obstruction and damage affected the proportion of pulmonary tobramycin reaching the periphery. The proportion of tobramycin reaching the lungs was small and variable.(ABSTRACT TRUNCATED AT 250 WORDS)