Purpose: The objectives of this study are to describe the 4D treatment planning process for moving targets and to evaluate the dosimetric consequences caused by target motion during tomotherapy delivery. Method and Materials: A lung cancer patient was CT scanned using our standard 4D‐CT protocols on Phillips brilliance 64 slice CT scanner. A maximum intensity projection CT dataset was used for treatment planning. Tumor motion trajectory and amplitude was determined from the inhale and exhale phases of the 4DCT. A three‐dimensional motion pattern was created by scaling the breathing form to an average magnitude of the target motion. The three‐dimensional trajectory was programmed into the Washington University 4D Phantom to simulate target motion. Eight EDR2 films were loaded into a phantom for dosimetric verification. Measurements were performed on a static phantom and then repeated on the oscillating phantom. Experiments were repeated for their reproducibility. Films were analyzed using Hi‐Art TPS. Results: To quantify the dose errors, gamma analysis was performed. In static phantom study, >97% of the points passed with a gamma criteria of 3% and 3mm. When the same criterion was applied for the oscillating phantom, 15–25% of the points failed. When the gamma threshold was increased to 8% and the gamma analysis window was tailored to be within the CTV, >98% of the points passed indicating that the dose errors are of the order of 5% for majority of the points. In order to find the maximum dose errors, the tolerance was increased until 100% of points passing the criteria. Conclusion: 4D treatment planning process for helical tomotherapy is discussed. Gamma analysis revealed that, the dose errors are of the order of 5% for majority of the points and the maximum dose error within the CTV was 11%. This work is supported in part by Tomotherapy Inc.