Tachycardia is a known adverse effect during clozapine treatment. However, prevalence reported differs widely between studies and hitherto there are no studies comparing clozapine-treated patients with a similar control group. The present study was carried out to assess the prevalence of tachycardia in patients treated with clozapine and antipsychotic long-acting injections (LAI). Data on heart rate (HR), concomitant medication, and relevant anthropometric and laboratory measurements were collected for all clozapine-treated patients (n=174) in a defined catchment area and compared with data on patients treated with LAI (n=87). In total, 33% of patients on long-term clozapine treatment had tachycardia (HR>100) compared with 16% in the LAI group (P<0.001). The mean HR was 91 in the clozapine group and 82 in the LAI group (P<0.001). Clozapine dose correlated with HR. The majority of patients with HR more than 100 received no specific treatment for tachycardia. In conclusion, the prevalence of tachycardia was twice as high in patients treated with clozapine as in a similar patient group with severe schizophrenia spectrum disorder. The tachycardia was in many cases clinically unnoticed. Tachycardia during antipsychotic treatment is a common phenomenon that must be monitored for actively and, when noticed, further investigated and treated.