The aim was to determine hemato-biochemical changes and rapid diagnosis of Theileria annulata in naturally infected crossbred cows. Blood samples from lactating crossbred cows (n=40) between 3 and 7 years of age and showing clinical signs of tropical theileriosis were collected, with or without anticoagulant, and analyzed for tropical theileriosis by direct smear, direct blood polymerase chain reaction (PCR) detection of merozoite-piroplasm surface antigen (Tams1) gene specific amplicon, estimation of hematological and biochemical parameters. Healthy crossbred cows (n=6), examined free from hemoprotozoan infections were included as control. The infected crossbred cows revealed significantly (p<0.001) lower values of total erythrocytic counts (4.46±0.2 × 10(6)/µL), hemoglobin (Hb 6.025±0.39 g%), packed cell volume (17.05±1.1%), mean corpuscular volume (37.94±1.70 fL) and mean corpuscular Hb (13.5±0.48 pg; p<0.002) compared with healthy control. The serum samples of infected cows revealed profound (p<0.05) hyponatremia (Na 133.21±2.36 mEq/l) and hypocalcemia (Ca 8.39±0.34 mg%). Infected crossbred cows showed a significant increase (p<0.05) of mean serum activity of alanine aminotransferase (61.45±13.36 U/L), aspartate aminotransferase (146.1±20.97 U/L), blood urea nitrogen (28.26±3.90 mg%), creatinine (1.55±0.13 mg%), direct bilirubin (0.33±0.04 mg%; p<0.001) and lactate dehydrogenase (3001.32±167.0 U/L; p<001). Blood direct PCR revealed a 721-bp fragment amplified from the target gene encoding 30-kDa major merozoite surface antigen of T. annulata using specific primer pairs. This assay was positive for all the infected animals. The assessments of hemato-biochemical parameters in T. annulata infected crossbred cows may be useful in understanding disease pathogenesis, prognosis and corrective measures for supportive therapy. Moreover, blood direct PCR can reliably be used for rapid detection of T. annulata in conjunction with microscopic examination.