Background: Thrombocytopenia is an abnormal decrease in platelet numbers and is a major detrimental side effect of chemotherapy or of cancer itself. Thrombocytopenia can lead to hemorrhage in vital organs, which is particularly a problematic specifically in solid cancer patients, and has a substantial negative effect on cancer patients’ quality of life (QOL). Thrombocytopenia ranges from mild bleeding from small blood vessels to severe bleeding from large blood vessels. Objectives: The present study aimed to describe the effect of chemotherapy regimens and doses on thrombocytopenia incidence and severity among solid cancer patients with thrombocytopenia. Materials and Methods: This retrospective observational study was conducted in Penang Hospital, a government hospital on Penang island, and included 341 cancer patients with thrombocytopenia who were admitted between 2003 and 2009. The main statistical tests used were the Chi-square test and logistic regression. The level of significance was set at P < 0.05. Results: Of the 341 patients included, 21 (6.2%) had thrombocytopenia before chemotherapy and the remaining 320 (93.8%) developed it after chemotherapy. The majority of patients had moderate thrombocytopenia (n=172; 53.8%), followed by mild (n=97; 30.3%), and severe thrombocytopenia (n=51; 15.9%). FEC, 5-FU+5-FU, Docetaxel and Cisplatin regimen had strong associations and correlation with thrombocytopenia incidence and severity, but the associations and correlations for thrombocytopenia severity were stronger than those for incidence. The dosage of 5-FU, cyclophosphamide, docetaxel and cisplatin plays a critical role in thrombocytopenia incidence and severity. Conclusion: Monitoring and determination of hemoglobin levels for cancer patients treated with FEC, 5-FU+5-FU, Docetaxel, and Cisplatin specifically with high doses must be emphasized and a focus of particular attention.