The aim of this study was to evaluate how lung function and growth changed over time in children with cystic fibrosis (CF) colonized with pseudomonas aeruginosa (Pa) compared with those free of the organism. A total of 192 children attended our cystic fibrosis clinic between 1982 and 1992. Sixty-two of these had three or more annual assessments for lung function, and 117 had three or more annual assessments for height and weight. When lung function was expressed as a standard deviation score (SDS), forced expiratory volume in 1 second (FEV1) and forced expiratory flow at 25% of vital capacity (FEF25) decreased significantly more with respect to height in colonized compared with noncolonized children: FEV1, -0.052 verses -0.015 SDS/cm (P < 0.05); FEF25, -0.060 verses -0.007 SDS/cm (P < 0.05); forced vital capacity (FVC), -0.034 versus -0.012 (NS). In actual values those patients colonized with Pa increased their FEV1 by 16.4 versus 31.6 mL/cm (P < 0.01); FVC by 28.8 versus 41.4 mL/cm, P < 0.01; and FEF25 by -0.001 versus 0.015 L/s/cm, P < 0.01. In terms of height, colonized children grew at 5.63 versus 6.96 cm/yr, P < 0.001, and height SDS decreased in colonized compared with noncolonized children at -0.031 verses 0.08 SDS/yr, P < 0.05. Clinically, most children with CF, with or without Pa, grew within +/- 1 SD of the norm for weight and height. However, in terms of lung function despite optimum pulmonary management colonized children deteriorated significantly faster.