OBJECTIVE - We previously found that microalbuminuria (MA) is present in 14% of patients with long-standing cystic fibrosis-related diabetes (CFRD). However, others have reported much higher rates of MA in CF patients with and without diabetes (32-67%), suggesting this test is not sufficiently specific for diabetic nephropathy screening in CF. We investigated transient (TMA) and persistent (PMA) microalbuminuria in CF patients to resolve these contradictory findings. RESEARCH DESIGN AND METHODS - We reviewed 1,449 outpatient urinary albumin measurements from 467 patients aged ≥10 years, which were collected over a decade. TMA was defined as a single episode of MA that subsequently was resolved. PMA was defined as two consecutive or two out of three consecutive measurements in the MA range. RESULTS - The prevalence of TMA that subsequently was resolved in CF patients was similar to the general population. It was found in 7.6% of patients, including 5% of youth (aged 10-17 years) and 9% of adults. PMA was found in 6.1% of the overall CF population, including 2% of youth and 8% of adults. The odds of PMA were increased sevenfold in patients with CFRD (95% CI 2.5-20, P = 0.0002) and 48-fold in patients with both CFRD and organ transplant (95% CI 13-177, P < 0.0001). The five patients with PMA in the absence of CFRD or transplant included two youths with presumed benign orthostatic MA and three adults with hypertension. CONCLUSIONS - The spot urine albumin-to-creatinine ratio is specific enough to be a valid screening test for diabetic kidney disease in CFRD.