Most of the non-obstructive azoospermia (NOA)-patients have only focal spermatogenesis which results in insufficient numbers of spermatozoa to reach the ejaculate. In ≈50% of these NOA-patients testicular sperm extraction (TESE) is successful and intracytoplasmic sperm injection (ICSI) is pursued. We studied whether (i) spermatogenesis can be evaluated by defining the ratios between Sertoli cells, pachytene spermatocytes and spermatozoa in a testicular cell suspension, and (ii) these ratios are associated with the outcome of fertility treatment. A retrospective cohort study was conducted between June 2007 and August 2012. In this period, 441 consecutive ICSI-TESE cycles were performed in 212 couples. For each TESE biopsy, the ratios between Sertoli cells, pachytene spermatocytes and spermatozoa were calculated. A control population of 32 vasectomized men was used to define cut-off values for complete spermatogenesis. Based on the pachytene to sperm ratio (P/Sp) and number of spermatozoa per 100 Sertoli cells (#Sp/100SC) groups were defined as complete spermatogenesis, hypospermatogenesis and partial maturation arrest (MA). Validation of the cytological diagnoses was performed by comparing the results of cytology to the histological evaluation of spermatogenesis in 40 cases. In 92.5%, a perfect match was observed and in the three remaining cases cytology corresponded well with the results of TESE. Couples with complete spermatogenesis have a higher ongoing pregnancy rate after the first treatment cycle compared to couples with hypospermatogenesis (34 vs. 16%; p = 0.02) and partial MA (34 vs. 19%; p = 0.11). In conclusion, pachytene spermatocytes, spermatozoa and Sertoli cells can be easily identified and counted in a cell suspension and their ratios can be successfully used to diagnose the level of spermatogenic impairment. This pilot study indicates that once successful spermatozoa retrieval is achieved, treatment outcome declines when spermatogenesis is impaired in NOA. The predictive value of cytological evaluation of spermatogenesis has to be established in a future prospective trial.