OBJECTIVE • To understand the effects of temperature, humidity and season of year on 24-h urine composition in patients with nephrolithiasis. PATIENTS AND METHOD • A retrospective review was performed of patients evaluated at four metabolic stone clinics. • Multivariate linear regression models examined the relationship between mean temperature, average humidity, season of year and 24-h urine composition. • Multivariate models adjusted for known risk factors for stone disease. • Mean temperature and average humidity data were obtained from http://www. weatherunderground.com based on patient-provided addresses. RESULTS • A total of 599 patients were included in the study, comprising 239 women and 360 men with a mean age of 53.6 years ( SD 15.0). • Mean temperature was 16.9 ° C ( SD 4.8, range-21.1 to 38.3 ° C) and average humidity was 58.1% ( SD 23.5, range 11-100%). • On multivariate linear regression, increasing temperature was associated with increasing urine calcium ( β =11.3, 95% CI 2.2-20.0), super-saturation of calcium oxalate ( β =0.6, 95% CI 0.2-0.9), super-saturation of calcium phosphate ( β =0.14, 95% CI 0.03-0.2), and decreasing urine sodium ( β =-5.2, 95% CI-10.3 to-0.1). • As seasons become warmer (i.e. from winter to autumn to spring to summer), changes were increased urine volume ( β =0.09, 95% CI 0.01-0.2) and decreased super-saturation of calcium phosphate ( β =-0.2, 95% CI-0.3 to-0.03). • There were no associations between quintile of humidity and any 24-h urine constituents. CONCLUSIONS • Increasing temperature may increase stone risk by increasing urine excretion of calcium, and the super-saturation of calcium oxalate and calcium phosphate. • These findings were independent of humidity and of season of year. • This appears to be related to a physiological impact of temperature itself, rather than to geographic location.