Objectives:  To investigate the natural courses of mild, moderate and severe idiopathic oligozoospermia, and which factors or semen variables were of utmost importance in predicting the courses. Methods:  A total of 208 men (age 29–47 years) who were diagnosed with mild, moderate and severe idiopathic oligozoospermia in a 9‐year‐period between January 2000 and December 2008 were followed up for more than 6 months. Results:  Overall, 16 (24.6%) of 65 patients with severe oligozoospermia developed azoospermia, whereas two (3.1%) patients with moderate oligozoospermia developed azoospermia and none of the patients with mild oligozoospermia developed azoospermia. Initial follicle stimulating hormone level and testicular volume between the subgroups were significantly different (P = 0.0071 and 0.0039, respectively). The subgroup of patients who became azoospermic (n = 18) showed statistically significant differences in terms of body mass index and the level of prolactin (PRL) from the subgroup that maintained the initial lingering sperm count (n = 190; P = 0.0086 and 0.0154, respectively). As the vitality of semen variables increased 1%, the risk of progression to azoospermia diminished by 0.892‐fold, according to Cox's proportional hazards model analysis. A receiver operating characteristic curve analysis showed that the area under the curve was 0.755 and the sperm concentration value with the highest sensitivity and specificity was the reference value of 3–5 million/mL, with a sensitivity of 0.746 and specificity of 0.711 (P = 0.01). Conclusions:  Patients with severe oligozoospermia should be warned of the possibility of becoming azoospermic and hence sperm freezing should be encouraged as early as possible.