Personal ultraviolet radiation (UVR) dosimetry is important because sunlight is the most important risk factor for skin cancer and a risk factor for some eye diseases and immunosuppression and related disorders. Integrating devices, such as polysulphone film dosimeters, are generally used. To measure the exact dose at specific times, we have developed a personal electronic UVR dosimeter that makes time-stamped measurements. It has a sensor with an erythema action spectrum response and a linear sensitivity (dose-response) with no offset. The sensor has cosine response, and the dosimeter can cope with environmental conditions such as rain, temperature and dirt. It can be programmed to measure with different time intervals and save the average of a specified number of measurements in the memory that can store 32 000 time-stamped measurements. It is small (36 x 28 x 13 mm), weighs 14 g and can work for 4 months without maintenance. It is worn on the wrist, is equipped with a watch showing the time and may thus be used in large-scale studies. The sensitivity can change by 10% due to temperature changes from -5 to 40 degrees C. The UVR dosimeter sensitivity is 0.09 standard erythema doses (SED)/h and the difference in total received dose during 7 days between a Solar Light 501 UV-Biometer (186 SED) and our UVR dosimeter was 3% and the median difference in daily total dose was 2.2%. The dosimeter provides unique possibilities. Examples of personal UVR measurements, data calculations and how they can be interpreted are given.