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Husbandry of exotic pets like ornamental fish demands for the same holistic veterinary care, which is provided in companion animal medicine. The aim of this study was to provide basic values for laboratory parameters of adult koi kept in outdoor ponds as a basis for evaluation of fish general health and detection of organic disturbances. In addition, seasonal and sexual differences were examined. Six natural and conventional ponds with long-lasting veterinary health care and good pond water quality for koi husbandry were selected. Out of these, 42 koi with equal sex distribution and different age and size range underwent parasitological and thorough clinical examination to confirm general health status. Plasma biochemistry, haematological parameters and enzyme activities were determined. Differences between sexes include significantly increased alanine transaminase (ALT) activities and total protein, albumin and triglyceride concentrations in females. Seasonal differences obtained in 32 (76.2%) koi between late spring and late summer mainly included significantly increased activities of aspartate transaminase (AST), creatine kinase (CK), and lactate dehydrogenase (LDH) during summer. Further seasonal differences in plasma biochemistry values and neutrophil fraction were minor. This study provides basic values for koi kept in outdoor ponds as a basis for good veterinary practice, which includes routine blood parameter evaluation in a plethora of species. Furthermore, potential influence of sex and season/water temperature are highlighted to prevent misinterpretation of different physiological states like reproductive activity as pathological conditions.