BACKGROUND Most methods used for the assessment of severe steatorrhea in cystic fibrosis (CF) are sensitive. In fact, the tests show their usefulness in a borderline zone of the results. Yet, the existing data related to acid steatocrit (AS) are still contradictory. Therefore, in the present study we have aimed to assess CF patients without or with mild steatorhea (<10 g/day) and evaluate the applicability of AS in such a subset of patients. METHODS In fifty-five CF patients, AS, fecal fat concentration (FFC) and fecal fat excretion (FFE) in 1-day stool collection were assessed from one to three times (149 samples). In 50 subjects, FFC, FFE, and AS were available for 3 subsequent days. It allowed for the calculations based upon 3-day fecal fat balance study. RESULTS The correlations between FFE/FFC and AS based upon 1-day stool collection, although statistically significant, were rather weak (r = 0.208, P < 0.011; r = 0.362, P < 0.000006, respectively). The correlations between FFE/FFC and AS based upon the 3-day stool collection, although stronger, did not show values a linear relationship (r = 0.394, P < 0.005; r = 0.454, P < 0.001, respectively). With no regard to the cut-off level for AS (10% and 20%), sensitivity, specificity, negative, and positive predictive values in the determination of abnormal FFC and FFE were not satisfactory. The flow charts describing the accuracy of AS to determine FFE and FC revealed a high level of uncertainty. CONCLUSIONS AS does not reflect in a reliable way FFE in CF patients without or mild steatorrhea. Its applicability in the assessment of FFC in such patients has therefore limited practical value.