OBJECTIVE: This study aimed to explore the effects of ambient temperature and relative humidity on insulin pharmacodynamics in adults with type 1 diabetes. MATERIALS AND METHODS: A three-way, cross-over, randomised study was performed in adults with type 1 diabetes mellitus (n = 10). The pharmacodynamics profile of a single dose of short-acting insulin (insulin lispro) was investigated, using a controlled environmental chamber, under three environmental conditions: (a) temperature: 15 degrees C and humidity: 10%; (b) temperature: 30 degrees C and humidity: 10%; and (c) temperature: 30 degrees C and humidity: 60%. A euglycaemic glucose clamp technique ensured constant blood glucose of 100 mg/dL (5.5 mmol/L). The following pharmacodynamic endpoints were calculated: maximum glucose infusion rate (GIRmax ), time to GIRmax (tGIRmax ), total area under the curve (AUC) for GIR from 0-6 hours (AUCGIR.0-6h ), and partial AUCs (AUCGIR.0-1h , AUCGIR.0-2h and AUCGIR.2-6h ). RESULTS: Higher temperature (30 degrees C) under 10% fixed humidity conditions resulted in greater GIRmax (P = 0.04) and a later tGIR.max (P = 0.049) compared to lower temperature (15 degrees C). Humidity did not affect any pharmacodynamic parameter. When the combined effects of temperature and humidity were explored, tGIR.max (P = 0.008) occurred earlier, with a lower late insulin pharmacodynamic effect (AUCGIR.2-6h ; P = 0.017) at a temperature of 15 degrees C and humidity of 10% compared to a temperature of 30 degrees C and humidity of 60%. CONCLUSIONS: High ambient temperature resulted in a greater insulin peak effect compared to low ambient temperature, with the contribution of high relative humidity apparent only at high ambient temperature. This suggests that patients with type 1 diabetes mellitus who are entering higher environmental temperatures, with or without high humidity, could experience more hypoglycaemic events.