OBJECTIVE - Cystic fibrosis (CF)-related diabetes (CFRD) is associated with declining pulmonary function and increased mortality. During oral glucose tolerance testing (OGTT), CFRD is defined by 2-h plasma glucose (PG2).We hypothesized PG elevations during OGTT resolving by 2 h, not meeting CFRD criteria, influence pulmonary function in CF. Thus we investigated the frequency of elevated 1-h OGTT PG (PG1) and its relationship with pulmonary function. RESEARCH DESIGN AND METHODS - Retrospective review of OGTTs was performed between August 2005 (annual screening initiation) and June 2008 at Children's Hospital of Philadelphia CF Center. First-time,well stateOGTTs (PG0, PG1, PG2) were analyzed. Additional data collected were: percent predicted forced expiratory volume in 1 s (FEV 1), BMI percentile, lung bacterial colonization, age, and sex.OGTTs were categorized as normal (PG2<140mg/dL), impaired glucose tolerance (IGT) (PG2 140-199 mg/dL), CFRD (PG2 ≥200 mg/dL), and indeterminate glycemia (INDET) (PG1 ≥200 mg/dL and PG2 <140 mg/dL). Frequency of PG1 ≥140 but <200 mg/dL was also noted. Multivariable linear regression was used to assess associations between percent predicted FEV 1, BMI percentile, and OGTT PG. RESULTS - OGTTs (101) were available (59 male/42 female; age 5.8-22 years, percent predicted FEV 1 = 94.5 ± 18%, BMI percentile = 52 ± 25%). With the use of PG2, 91 OGTT were normal, eight were IGT, and two were CFRD. With the use of PG1 (n = 89), 39 OGTT were normal, 36 were PG1 ≥140 <200 mg/dL, and 14 were PG1 ≥200 mg/dL. PG1 was negatively associated with percent predicted FEV 1, adjusting for BMI percentile (P = 0.009, R 2 0.13). Percent predicted FEV 1 was not associated with PG0, PG2, age, sex, or lung bacterial colonization. CONCLUSIONS - PG elevations at nontraditional OGTT times are common in CF. The association of increasing PG1 with worse pulmonary function suggests early PG abnormalities may be deleterious or an early marker for worsening disease and will be missed if CFRD diagnosis focuses on PG2.