Thirty-two infants with cystic fibrosis (CF) had pulmonary function testing and chest radiographs at the time of diagnosis and on average 1 year later, when they had no acute respiratory symptoms. At diagnosis, 14 of 32 infants had respiratory symptoms (RESP) and 18 did not have respiratory symptoms (NRESP). There were no significant differences in age, weight, or length between the RESP and NRESP groups. At diagnosis, the RESP group had significantly lower forced expiratory flows compared to the NRESP group (41 +/- 32% vs. 98 +/- 48% predicted); however, there were no significant differences in functional residual capacity or chest radiographic scores. Between diagnosis and follow-up, the NRESP group had no significant change in pulmonary function but a decline in chest roentgenographic (CXR) scores (22 +/- 2 to 21 +/- 2). For infants in the RESP group, there were no significant changes in FRC or CXR score. Maximal expiratory flow at functional residual capacity (Vmax FRC) rose from diagnosis to 1 year follow-up (41 +/- 32% to 74 +/- 27% predicted; P < 0.002); however, at follow-up flows for the RESP group remained significantly lower than flows for the NRESP group (74% vs. 113% predicted; P < 0.0005). For the 32 infants with CF, there was significant correlation between percent predicted Vmax FRC at follow-up and at diagnosis (r = 0.47; P < 0.02). Those infants with lower percent predicted flows at diagnosis were more likely to have lower percent predicted flows 1 year later.(ABSTRACT TRUNCATED AT 250 WORDS)