Radiation in the near infrared over a limited range of wavelengths about 1660 nanometers is projected on a portion of the body, for example, the ear, of the patient. The resulting radiation emitted by the portion, either scattered from the portion or transmitted after absorption and scattered by the portion, is process to derive an expression of the resulting radiation as a function of the wavelength. The second derivative of this function over a very narrow range of this function between 1640 and 1670 nanometers is expanded and the glucose concentration is determined from the magnitude, or intensity, of the scattered or transmitted radiation at the maximum or minimum point of this derivative. Apparatus for non-invasive determination of glucose concentration in the patient. Radiation in the near infrared is transmitted through a first fiber-optic radiation conductor to the outer surface of a portion of the patient's body, penetrating into the portion. A second fiber-optic conductor transmits the resulting radiation emitted from the portion to data processing means which produces the desired second derivative as a function of the wavelength of the incident radiation. The processor includes a spectrum analyzer which produces a spectrum of the resulting radiation.