The current study aims to determine the prevalence of bacterial urinary tract infection (UTI) and estimate the serum levels of C3 and C4 in UTI patients. This study had been included 350 samples (blood and urine) obtained from outpatients suspected of having UTI from Baquba-Teaching Hospital. The detection of causative Bacteria was done using an automatic VITEK 2 system. A sample of 350 urine was taken, and after diagnoses, it found that only 205 (58.5%) had UTI; 123 (as 60 %) with a positive culture of bacteria, and 82 (as 40%) with negative culture of bacteria. Among the Gram-negative bacteria, mostly isolated were Escherichia coli 44 (resembles 35.7%), K. Pneumonia 18 (resembles 14.6%), P. aeruginosa 8 (resembles 6.5%) 2 (resembles 1.6 %) which was diagnosed as Burkholderia cepacia. Gram-positive organisms included 51 (as 41.4%) of total positive results, divided into 37 isolates (30.08%) of Staphylococcus spp. and 14 isolates (11.3%) of Enterococcus spp. Immunological parameters, which were included the determination of serum levels of C3 with C4 showed that lower levels of both C3 and C4 in UTI patients compared with the control group (18.48±1.3 versus 19.4 ± 2.9pg/ml) and (0.739±0.257 versus 1.205±0.259 pg/ml) respectively. In conclusion, E. coli, K. pneumoniae and S. aureus were found to be the highly isolated bacteria in UTI cases, with decreased levels of C3 and C4 in serum in UTI patients.