A portable electronic spirometer comprising: (a) a tubular mouthpiece with - a proximal opening for insertion into the mouth of a user, - a distal opening, - a main fluid channel extending between the proximal opening and the distal opening, - a first lateral opening, - a second lateral opening positioned at a longitudinal distance to the first lateral opening, and - a flow restrictor positioned in the main fluid channel between the first and the second lateral opening (6 and 7); and (b) a main body with - a first fluid opening connectible with the first lateral opening of the mouthpiece, - a second fluid opening connectible with the second lateral opening of the mouthpiece, - a bypass fluid channel extending between the first and the second fluid opening (10 and 11), - a MEMS-based thermal fluid flow sensor positioned at the bypass fluid channel for generating a signal in response to the fluid flow in the bypass fluid channel, - an acceleration sensor which is different from the MEMS-based thermal fluid flow sensor (13, 13.1, 13.2), and - a microcontroller connected with the fluid flow sensor for calculating the fluid flow from the signal generated by the flow sensor, and optionally - a communication means for the exchange of data related to the fluid flow generated by the spirometer, preferably by the microcontroller of the spirometer. (a) a tubular mouthpiece with - a proximal opening for insertion into the mouth of a user, - a distal opening, - a main fluid channel extending between the proximal opening and the distal opening, - a first lateral opening, - a second lateral opening positioned at a longitudinal distance to the first lateral opening, and - a flow restrictor positioned in the main fluid channel between the first and the second lateral opening (6 and 7); and - a proximal opening for insertion into the mouth of a user, - a distal opening, - a main fluid channel extending between the proximal opening and the distal opening, - a first lateral opening, - a second lateral opening positioned at a longitudinal distance to the first lateral opening, and - a flow restrictor positioned in the main fluid channel between the first and the second lateral opening (6 and 7); and (b) a main body with - a first fluid opening connectible with the first lateral opening of the mouthpiece, - a second fluid opening connectible with the second lateral opening of the mouthpiece, - a bypass fluid channel extending between the first and the second fluid opening (10 and 11), - a MEMS-based thermal fluid flow sensor positioned at the bypass fluid channel for generating a signal in response to the fluid flow in the bypass fluid channel, - an acceleration sensor which is different from the MEMS-based thermal fluid flow sensor (13, 13.1, 13.2), and - a microcontroller connected with the fluid flow sensor for calculating the fluid flow from the signal generated by the flow sensor, and optionally - a communication means for the exchange of data related to the fluid flow generated by the spirometer, preferably by the microcontroller of the spirometer. - a first fluid opening connectible with the first lateral opening of the mouthpiece, - a second fluid opening connectible with the second lateral opening of the mouthpiece, - a bypass fluid channel extending between the first and the second fluid opening (10 and 11), - a MEMS-based thermal fluid flow sensor positioned at the bypass fluid channel for generating a signal in response to the fluid flow in the bypass fluid channel, - an acceleration sensor which is different from the MEMS-based thermal fluid flow sensor (13, 13.1, 13.2), and - a microcontroller connected with the fluid flow sensor for calculating the fluid flow from the signal generated by the flow sensor, and optionally - a communication means for the exchange of data related to the fluid flow generated by the spirometer, preferably by the microcontroller of the spirometer.