Human health is affected by weather elements such as temperature, rainfall, relative humidity, wind direction and others. The aim of this study was to examine the effect of rainfall, temperature and relative humidity on the occurrence of malaria, typhoid and tuberculosis in Onitsha North LGA, Anambra State, this was achieved through the following objectives; determine the pattern of occurrence of rainfall, temperature and relative humidity, determine the pattern of distribution of cases of malaria, typhoid and tuberculosis, determine the pattern of distribution of cases of malaria, typhoid and tuberculosis and to determine the seasonality of malaria, typhoid and tuberculosis (2004-2013).Records of rainfall, temperature and relative humidity was obtained from Anambra State NIMET office, Awkaand the records of malaria, typhoid and tuberculosis cases from the selected hospitals(2004-2013) were also used to carry out this study. Time series analysis was used to analyze the trends of each of the data collected. Pearson Product Moment Correlation analysis was used to analyze the relationship between the occurrence of the three diseases and the climatic parameters. The seasonal occurrence of the diseases was analyzed by dividing the year into cold dry season, hot dry season and rainy season. The analysis showed that rainfall, relative humidity and malaria exhibited decreasing trends while temperature, typhoid and tuberculosis exhibited increasing trends within the period 2004-2013. The correlation analysis indicated that malaria, typhoid and tuberculosis are influenced differently by the various weather elements. Rainfall showed a significant relationship with malaria with an r value of 0.646 and an insignificant relationship with typhoid and tuberculosis having r values of 0.519 and -0.341 respectively, while temperature showed no significant relationship with any of three diseases with malaria having an r value of - 0.194, typhoid; -0.253 and tuberculosis; -0.102. Relative humidity showed a significant relationship with malaria and typhoid with r values of 0.852 and 0.807 respectively and an insignificant relationship with tuberculosis with r value of -0.427. The seasonal analysis showed that malaria and typhoid had their highest number of occurrences in the rainy season with 84.87 and 2.46 cases and their lowest in the cold dry season with 76.62 and 1.75 cases relatively, while tuberculosis had its highest number of occurrence in the cold dry season with 16.97 cases and the lowest reported case reported in the rainy season with 11.31 cases. The hypothesis was tested and accepted in the case of tuberculosis and rejected in the case of malaria and typhoid in the study area. It was recommended that all the three tiers of government should come up with enlightenment programs should be conducted to educate people on the effect of climate change on the occurrence of diseases; to also provide safe and adequate drinking water for the masses; and provide free mosquito nets for the residents of the area. The people on the other hand should improve on their sanitation in their environment.