Background: Chemotherapy often leads to encouraging responses in lung cancer. But, in the course of the treatment, resistance to chemotherapy ultimately limits the life expectancy of the patient. We aimed at investigating if treatment with cisplatin alters the intracellular Ca2+-homeostasis of lung cancer cells and how this may be related to cisplatin resistance.Methods: The squamous cell lung carcinoma cell line EPLC M1 and the small cell lung cancer cell line H1339 were exposed to cisplatin analogue to the in vivo pharmacokinetics. Changes in the cytoplasmic Ca2+-concentration ([Ca2+]c) were recorded using fluorescence microscopy. Protein expression was quantified using immuno-fluorescence and Western Blot analysis. Changes in gene expression were accomplished by small-interfering (si) RNA techniques.Results: Four “cycles” of cisplatin led to low level resistance in EPLC and H1339 cells. In the low level resistant cell clones, the Ca2+-content of the endoplasmic reticulum (ER) was decreased. In low level resistant EPLC cells, this was correlated with an increased expression of the inositol-1,4,5-trisphosphate receptor (IP3R). Inhibiting the increased expression of IP3R using siRNA, the low level resistance could be reversed. In low level resistant H1339 cells, the decreased Ca2+-content of the ER was correlated with a decreased expression of sarco/endoplasmic reticulum Ca2+-ATPases (SERCA). Decreasing the expression of SERCA in naïve H1339 cells resulted in low level cisplatin resistance.Conclusion: We conclude that cisplatin therapy leads to a decreased Ca2+-content of the ER thereby inducing low level resistance. This is caused by upregulation of the IP3R in EPLC and decreased expression of SERCA in H1339 cells.