Prevalence of genetic defect of UGT 1A1 in population is appreciable, the homozygous carriage takes place from 5 to 10% in different regions, and the heterozygote carriage reaches 40—45%. Diagnostics of the Gilbert’s disease (GD) also leads a lot of time often to hyper diagnosis of chronic hepatitis, or quite often patients not been fully investigated. Aim. Research objective: development of algorithm of diagnostics of illness of Gilbert. Material and methods. Research included 88 people (43 (49%) of patients with GD, 25 (28%) patients with chronic hepatitis and 20 (23%) patients of control group. To all patients assays with a low-calorie diet and fenobarbital, genetic inspection by means of polimerase chain reaction of the pro-motor UGT1 Promotor (ta) n area were carried out. Results. When carrying out dough with a hypohigh-calorie diet sensitivity and specificity of dough when rising level of a bilirubin more than on 21.4 µmol/l made 73,8% and 100% respectively. When carrying out assay with fenobarbital normalization of level of a bilirubin is noted at 68 patients (77,2%), sensitivity and specificity of dough made 82,9% and 63,3%. Consecutive application of two functional tests enlarges sensitivity and specificity of diagnostics. Change of functional assays became perceptible at homozygous and heterozygosis patients. Conclusion. When carrying out combinations of assay with a low-calorie diet and fenobarbital sufficient results on diagnostic level are received.